Requirements, Types, Choices, Build or Buy, Safety

Modern small to medium size HeNe tubes require an operating voltage between
about 900 and 2,500 VDC at 3 to 6 mA and a 5 to 12 kV starting voltage (but
almost no current). Precise values depend on the size and construction of the
tube. This assumes something in the .5 to 10 mW range. Larger tubes will
required greater voltage and current (e.g., 5,000 VDC at 8 mA, 15 kV to start)
but are powered in basically the same way as their smaller siblings. However,
a 250 mW monster as well as some very old tubes will likely have significantly
different or additional power requirements.

A few HeNe lasers - usually larger or research types - have used a radio
frequency (RF) generator - essentially a radio transmitter to excite the
discharge. This was the case with the original HeNe laser but is quite
rare today given the design of internal mirror HeNe tubes and the relative
simplicity of the required DC power supply as described later in this chapter.

Unlike laser diodes, the HeNe drive is not nearly as critical to performance
and tube life. :-) Therefore, even if you do not have a datasheet for your
tube, you can probably guess fairly closely as to its requirements and then
optimize performance based on optical output alone.

Then again, see Oops!
HeNe Laser Tube Meltdown. This is what happens if your tube and power
supply aren't matched. Just kidding. I actually suspect that someone got
just a bit carried away with a glass working torch to create this sculpture
but it got your attention, huh? :) (This mantlepiece sculpture was created
by Wayne Crowther (esoteric@pacifier.com).)

If you just want a working laser, buying a power supply may be worth the
money - these may be available for as low as $10 or less.
(All Electronics had a recent
special on a high quality power supply 'brick' but that (not surprisingly)
must have been sold out as it has disappeared from their on-line catalog).
More typical surplus prices would be $25 for a power supply suitable for 1
to 2 mW tubes and up to $100 for 5 to 10 mW tubes. Expect to pay 2 or 3
times as much for new power supplies. Laser surplus places like
Meredith Instruments and
MWK Laser Products may also
have low cost HeNe laser power supplies.

With these, at most you will have to add a ballast resistor and power line or
battery connections. Models are available that run off either low voltage
DC (regulated is desirable) or 115 or 230 VAC.

Kits are also available but may not be any cheaper and are not necessarily
as well designed as surplus commercial units. Two popular suppliers include
Herbach and Rademan Co. and
Edmund Scientific (complete HeNe laser
kit). Laser surplus places may also have power supply kits.

The complete laser and optics assemblies from supermarket checkout UPC
scanners and other similar devices are often available at very reasonable
prices - $25 to $100 - which includes the HeNe laser tube, power supply,
various lenses and mirrors, scanner motor or galvo, and other neato stuff.
Just the HeNe tube and power supply may be available for even less. Since
these systems are being converted over to diode lasers, the HeNe components
are just junk to the scanner manufacturer - but a bonanza to the hobbyist
and experimenter!

HeNe tubes and power supplies are quite frequently offered by people
posting on the sci.electronics hierarchy, sci.optics, alt.lasers, or other
USENET newsgroups as well as other on-line discussion groups, technical
forums, and bulletin boards. Prices are often very low but of course you
may have no way of knowing who you are dealing with. See the section:
On-Line Sources (Non-Commerical).

Whether you have constructed your own power supply, are testing an old one,
or just checking out a newly acquired HeNe tube, SAFETY must come first:

See the section: Laser Safety with respect
to the optical hazards associated with lasers. While not in the metal cutting
class, careless use of higher power HeNe lasers, especially, can result in
permanent damage to vision. Many are only Class II or Class IIIa but some are
well into the Class IIIb range.

HeNe laser power supplies utilize high voltage at low current. The few mA
needed to operate the HeNe tube or even a 10 kV starting pulse tube can
certainly give you an unpleasant shock but will not likely be enough to do
lasting damage. However, there may be much more current actually available
and there is always the possibility of collateral damage:

If the power supply is home-built, it may use grossly oversize components
because they are less expensive and more readily available than what is
exactly needed - or the unit has been conservatively designed to run much
larger lasers. Therefore, while the HeNe tube may only require 5 mA, the
supply may be capable of dumping many times this amount into your poor
unsuspecting body. And if you come in contact with a live terminal, it will
take the shortest path to ground though a nice low impedance bag of mostly
water. :-)

For supplies running on 115 VAC, there will be some components directly
connected to the power line. These may be minimal but make sure they are
insulated and out of reach. That 115 VAC may not sound like much but it is
potentially far more dangerous than the 10 kV starting voltage.

Contact with the operating and/or starting voltages can result in a reflex
reaction which may have unfortunate consequences (as in you rip and burn
your flesh, and shock yourself simultaneously). In other words, the shock
may not hurt as much as the foot long gash and burnt impression in your arm.
Not to mention the damage to your disposition as you clean up the bits of
glass and electronic components now littering the basement floor :-(.

People working with lasers tend to do so in a darkened room. Under these
conditions especially, make sure all protective covers and insulating
barriers are in place and tape over exposed connections unless you actually
require access to the internal components.

Even after the laser is shut off, the filter capacitors in the power
supply as well as the capacitance of the HeNe tube itself can store a charge
for a long time - hours, maybe even days! (Even if there is a bleeder
resistor on the main supply, there is probably none on the starting
multiplier.) While this isn't likely to be lethal, a reflex reaction to it
can have similar consequences to touching the live supply, above. Where a
HeNe tube doesn't want to start, up to 10 kV or more may be sitting there
waiting to zap you!

Always discharge between the power supply output and/or HeNe tube terminals,
and between each of these and the earth Ground or common of the power supply
and case of the laser head (if not tied to the cathode) before touching
anything inside or even the esposed ends of the head's male Alden connector
if you just umplugged it. Use a set of well insulated leads and a total of
about 100K of resistance rated for high voltage (e.g., 5, 20K, 1 W resistors
in series). CAUTION: Omitting the resistors may result in annoying snaps
and sparks as well as possible damage to the equipment. To be absolutely
sure, do all the combinations more than once and then confirm that
everything is completely discharged with a voltmeter. Leaving a shorting
lead in place across the terminals while working inside is a good idea as
well. For laser heads with Alden connectors, I usually pull the connector
and poke its pins into some antistatic foam to discharge the laser head.
However, this doesn't discharge the pwoer supply. Note that even after
doing any or all of this, capacitors have been caught regaining a part of
their charge when no one was looking! :) Also see the section:
Adding a Bleeder Resistor to a HeHe Laser
Power Supply.

This is probably overkill (no pun...) for a laser you are using as a laser. :)
But if you are wiring up a system which is to be used for testing where it may
be necessary to go inside quite frequently, it may be possible to add a
high voltage bleeder resistor directly across the output. It will be need
to be both a high resistance (200M or more) and rated for the peak starting
voltage (10 kV or more). A series string of 10, 20M, 1/2 W resistors in
an insulated (plastic or glass) tube would be a suitable substitute.
The higher the resistance, the longer it will take to discharge the power
supply and HeNe tube capacitance. However, the minimum resistance that can
be used will depend on the amount of starting current available. For
'brick' power supplies, 200M should be acceptable. However, for voltage
multiplier based starters, a much higher resistance may be needed so as not
to load down the output excessively. Confirm that the amount of time needed
to start the largest tube to be used on the supply doesn't change
significantly with and without the bleeder before installing it permanently.

Construction Alternatives, Organization

If you still want to build your own, there are basically two approaches for
the operating voltage (AC line operated or high frequency inverter) and three
approaches for the starting voltage (diode/capacitor multiplier, pulse trigger
circuit, or high compliance design).

Here are six options for providing the operating voltage. These examples
assume an output of at least 1,800 VDC:

Use a 1,300 VRMS power transformer. Then, all that is needed is a
rectifier and filter capacitor.

Use a 700 VRMS power transformer with a 2 diode 2 capacitor voltage
doubler.

Use a lower voltage power transformer and a multi-stage voltage multiplier.
Up to 6 stages should be reasonably easy to construct.

Build a low voltage input inverter using a flyback transformer from a small
B/W or color TV computer monitor, or video terminal but running at lower
voltage than normal. These usually have a built in HV rectifier but you
will need a HV filter capacitor and ballast resistor. While rated at only
a mA or so for the CRT HV, more current should be available at reduced
voltage. With proper design, it is possible for there to be enough voltage
compliance to be self starting. See the section:
Sam's Inverter Driven HeNe Laser Power
Supplies.

You will probably need to modify these mostly by increasing the number of
turns on the output windings of the inverter transformer - which in itself
may be a challenge to maintain low capacitance and high voltage insulation.

I would recommend (1) or (2) if portability is not a big issue and you can
locate a suitable transformer. These are virtually foolproof (well, at least
as long as you don't fry yourself from the high voltage). For small tubes,
the design described in the section: Edmund
Scientific HeNe Laser Power Supply is about as simple as possible.

A typical configuration is shown below. As noted, the starting circuit may
be omitted in a high compliance design. The regulator is desirable but the
location shown (low-side series) is just one option. Without a regulator,
tube current will need to be set by controlling the power input and/or
selecting the ballast resistor.

The ballast resistor (Rb) may sound really boring but it is essential to
provide stability and limit the current to the value specified for your
particular tube. The most common value is 75K but slightly smaller and much
larger values may be used to match a particular HeNe tube to a power supply.

A higher voltage supply and larger ballast resistance will be more stable if
there is no built-in regulation. This results from the fact that the voltage
drop across the tube is relatively independent of tube current so it subtracts
out from the supply voltage. What is left is across the ballast resistor and
changes by a proportionally greater amount when the line voltage varies. The
smaller the ballast resistor, the more this will affect tube current.

You really do want the current to be close to that recommended for your tube
both to maximize the life of the tube and because maximum optical power output
is produced at the optimal current. Note that the HeNe tube specs will
include the recommended current. In some cases, this may not coincide with
maximum output power due to manufacturing tolerances, age/use, or other
factors. It may be acceptable to run such a tube on the current (probably
higher than spec'd) which maximizes output power but this would have to
be determined on a case-by-case basis and except for some very wide bore
(multimode) tubes, should never be higher than about 6.5 mA.

Setting the tube current can be accomplished by adjusting either the input
voltage to the power supply (if there is no current regulator) or the
regulator (if there is) and/or by selecting the value of the ballast
resistance. Excessive current is bad for the tube and will actually
result in decreased optical output (and more optical noise though you'd
probably never notice that by eye). If the current is too high by a factor of
2 or 3, there will be no output at all. It is not possible to pulse a HeNe
laser for higher power (though pulse drive is possible for modulation - see
the section: Pulse Type Drive and Modulation of
HeNe Tubes). Without any regulation, the value of the ballast resistor is
more critical and power line fluctuations will significantly affect tube
current though such variations may not matter for many applications.

Note that since the tube itself provides a relatively constant voltage drop
around its nominal current, a small change in line voltage will affect the
tube current to a much greater degree than would be expected by the percent of
the actual voltage change. The incremental change in tube current will be
closer to: delta(I) = delta(V)/Rb where Rb is the ballast resistor. This will
be on the order of 10 to 20 times more sensitive to input voltage changes than
with a straight resistive load. Thus, a regulator is often desirable.

The large (can) electrode of the HeNe tube is the cathode (-) end. It may run
if connected backwards but the small anode (supposed to be positive but now
incorrectly connected as negative) will get very hot since much of the power
dissipation is at the negative electrode due to positive ion bombardment.
The resulting sputtering may damage the mirror at that end and tube life will
likely be shortened.

I've autopsied an HeNe laser tube which had been ruined in a few minutes by
being driven from a Tesla coil based on an automotive ignition coil and relay
interrupter without rectification. The result was sputtering of the HR mirror
which was clearly visible once the mirror was removed. Except under very
special conditions, any HeNe laser power supply must produce a DC output that
is relatively well filtered at an operating current reasonably close to the
spec'd value for the tube. Anything else will result in reduced tube life and
less than optimal output power. Two exceptions are high speed pulsed drive
where the plasma never really gets extinguished between pulses (for laser beam
modulation) and RF excitation. For the purposes of making the laser lase, DC
is what is needed. :)

CAUTION: While most modern HeNe tubes use the mirror mounts for the high
voltage connections, there are exceptions and older tubes may have unusual
arrangements where the anode is just a wire fused into the glass and/or the
cathode has a terminal separate from the mirror mount at that end of the tube.
Miswiring can result in tube damage. See the section:
Identifying Connections to Unmarked HeNe Tube
or Laser Head if in doubt.

However, if you have trouble changing a light bulb, never fear - packaged
solutions are readily available at attractive prices. You don't need to build
your own HeNe laser power supply unless you have some special requirements, a
really strange HeNe tube, or just for the shear joy of creation!

Excitation of a laser using radio frequency or microwave power is possible and
has been used commercially for several types of lasers including HeNe and CO2.
(with patents granted on many others including ion lasers). However, you
aren't likely to get much just by sticking a HeNe laser tube in the family
microwave oven other than glass shards all over the place. :( Nor can you
insert your HeNe tube into the antenna coil connected to the 2 kW final amp
of your Ham radio transmitter.

The main problem is that you have to get the discharge intensity high enough
inside the bore - that 1 mm or so diameter capillary tube where the action
takes place. In a typical internal mirror HeNe tube, 98 percent or more of
the cross sectional area is outside the bore. Since this is also gas filled,
it will happily glow at equal or greater intensity than the bore, sucking up
power and getting very hot in the process - while contributing nothing to the
output. So, the light show may be spectacular (until the thing explodes), but
a laser beam is unlikely. However, this is a way of testing to see if a tube
has lost its gas fill, but stopping short of the 'explode' part. :) See the
section: How Can I Tell if My Tube is Good?.

This is also not a viable means of exciting a HeNe tube that no longer lases
due to gas fill or most other problems. A conventional DC power supply will
tell you that the tube is dead with a lot less hassle!

For an external mirror tube where the bore is accessible and separate from
the gas reservoir, direct coupling is possible by using a suitable matching
network from an RF generator. This can be made to work since the energy can
be forced to couple only to the gas inside the capillary. One reason to try
a scheme of this sort would be to power such a laser where one of the ballast
resistors has failed open and they are located inside the sealed tube (as with
some Spectra-Physics models).

The earliest HeNe lasers were RF excited in this manner but they had wide
bores and mediocre output power. If you are interested in trying your hand
at duplicating one of these, then sure, go for it! However, those lasers also
had problems with rapid helium diffusion apparently accelerated by the RF
fields applied to the glass so a sealed tube may be out of the question.

WARNING: The use of high power RF or microwave excitation creates an number of
additional safety hazards beyond just simple electrocution. Attempting to
drive a HeNe (or other laser) in this manner isn't something you should think
about experimenting with unless you have experience with this type of equipment
and are thoroughly aware of the associated risks and precautions involved.

Hey, with 'neon' in the name; how different can can a neon tube be compared to
a helium-neon laser?

Small discharge tubes are found in high-tech phones and other electronic
sculpture. A typical unit would be perhaps 2 feet long and 1/4" to 1/2" in
diameter. Modern power supplies for these tubes use an inverter to produce
high frequency high voltage AC (not DC as required by a HeNe laser tube).
Older ones will just use a mini version of a high voltage neon sign
transformer. Since large inside diameter (compared to HeNe lasers) neon tubes
don't require a special starting voltage, this will be lacking in any such
power supply.

While it may be possible to adapt portions of one of these to powering a HeNe
laser, this probably isn't worth the effort. A rectifier, filter, starting
voltage source, and ballast resistors would need to be added. With luck, the
ballast resistance alone could be adjusted to set the tube current. Otherwise,
additional circuitry possibly including regulation would be needed. However,
the high frequency inverter (or high voltage transformer) might be
salvageable. :)

Similar comments apply to power supplies for fluorescent (e.g., that defunct
camping lantern or obsolete laptop computer) and other high voltage discharge
lamps.

Note that the power supplies for regular neon signs consisting of 10s of feet
of glass tubing are, in addition to the reasons cited above, even more
inappropriate for almost any size HeNe laser - they are gross overkill in both
the voltage and current departments unless run at reduced input voltage on a
Variac. These are discussed in more detail below.

Using Commercially Available Power Supplies

Most commercial HeNe laser power supplies designed for incorporation into OEM
(Original Equipment Manufactured) equipment (as opposed to lab type adjustable
supplies) are affectionately called 'bricks' because they are solid black
rectangular blocks (dimensions vary) potted in Epoxy (or some equally
impenetrable resin) with wires sticking out. They are generally high
frequency inverter types operating either on 115 or 230 VAC, or low voltage
DC (typically 6 to 30 VDC). Their output is constant current-regulated over
a wide voltage compliance range. The current may fixed, adjustable via a
trim-pot, or very rarely, but an external resistor. If adjustable, the
range may be up to 2:1 or more. The voltage compliance range over which the
current regulation is assured is typically at least +/-10 percent of
voltage specifications, but is often larger. Internal power dissipation is
often what limits the actual capabilities of these power supplies.

Melles Griot HeNe Laser Head and Laser Drive Power
Supply Brick shows a small Melles Griot laser head with a compatible Laser
Drive power supply. This brick is likely similar to the Melles Griot model
05-LPM-379 with an output voltage compliance range of 1,150 VDC to 1,700 VDC.
It has a trim pot to set the current between about 4 and 6.5 mA providing
compatibility with almost any 1 to 2 mW HeNe laser head or (with an external
75 K ohm ballast resistor) HeNe tube.

Power supply bricks are compact, highly efficient (well, in a relative sort
of way as these things go!), and are generally very robust and reliable when
used as intended. The specifications usually claim to include protection
for *momentary* open and short circuit faults, (as well as repeated arcs
to ground, and low input voltage). Thus, they shouldn't self destruct as a
result of most reasonable screwups in wiring. (Of course, if you accidentally
attempt to power a 12 VDC unit from 230 VAC, the smoke *will* come out!).
However, bricks may fail if a laser head is not attached (or a hard-to-start
tube fails to start) after a few minutes or hours. Continuously attempting
to start without success is stressful for any HeNe laser power supply so
don't push your luck! And despite the supposed fault protection,
many seem to be not particularly immune even to high resistance shorts
as myself and others have found out the hard (expensive) way. DC input
types may not survive reverse polarity either. And, of course, being
fully potted, only in very rare cases are they useful for something other
than construction bricks or sailboat ballast following any internal failure.

The simplest approach to powering HeNe tubes is often to purchase this type
of power supply, either new or (more likely) surplus. With a little bit of
searching, they are readily available at attractive prices. Manufacturers
include Laser Drive (now part of Martek
Power), Power Technology, and Melles Griot. Aerotech used to
make these as well but has since gotten out of the HeNe laser business (but
you may still find their HeNe tubes and power supplies from surplus sources).

Cost may be anywhere from $10 to $200 or more depending on power capability,
whether it is new or surplus, and other factors. Typical surplus prices are
$10 to $35 for a unit capable of powering a .5 to 2 mW HeNe tube and $25 to
$100 for one suitable for a HeNe tube up to about 5 mW. Such power supplies
(and HeNe tubes to go with them) may even be offered by private individuals on
eBay and elsewhere, possibly at even lower prices. Of course, the usage
history, quality, and reliability (of both the equipment as well as the
seller) from such sources may be unknown.

The basic power supply specs will include the HeNe tube current and voltage
compliance range. The total voltage across your tube and ballast resistor at
the operating tube current must fall within this range. Since many of these
bricks are set for a fixed current and may not have any user accessible
adjustments (that is, without using a jack-hammer), the power supply and HeNe
tube current specs must be fairly closely matched. Look for types with an
adjustable current setting if possible. (Even if the specs or sticker on the
unit show a fixed current, there may actually be an adjustment that is either
obvious or protected by a removable plug since it is quite likely that the
manufacturer fabricates and pots one type module that can be tweaked for a
range of currents to reduce the need for separate inventory. However, without
confirmation from the manufacturer, there is no way to know for sure whether
adjusting it far from the specified current is acceptable and doesn't result
in excessive stress on the supply's circuitry.) Some may be jumpered for
2 or more fixed current settings.

In addition to power input and high voltage output connections, these supplies
may also have one or more of the following: logic level enable (may need to be
grounded or tied to a DC voltage), logic level output for indicator or
something else, auxiliary DC power output(s), CDRH turn-on delay loop (cut to
disable delay), HeNe tube cathode to ground return loop (cut to add current
meter or beam-on indicator LED). In many cases, there will be a wiring diagram
on the brick. If none is present, although there is no total standardization,
some color coding conventions are usually followed. See the section:
Common Color Coding of Power Supply Bricks.

Power supplies may also be packaged along with a small HeNe tube, ballast
resistor(s), and wiring as a complete laser optics assembly. These have become
available at very attractive prices as products like UPC scanners and laser
disc players have switched to over to the use of laser diodes. Since nearly
everything but a wall plug is likely included in such a package, and hopefully
the HeNe tube is matched to the power supply (with respect to current and
voltage compliance), this approach should result in a working laser with
minimal effort.

There are also many HeNe laser power supplies available that are not potted
but simply conformal (dip) coated or are just unprotected components on a
printed wiring board. (The potting or coating is desired to prevent high
voltage arcing or corona discharges.) These often spent their former life in
hand-held barcode or supermarket checkout scanners. I like the naked ones
because it is possible to reverse engineer them (I will pay shipping for
any of these (dead or alive) you might have so I can add their schematics to
the Laser FAQ!). However, their quality and reliability is often a total
unknown.

At the other end of the scale, lab quality HeNe laser power supplies in fancy
enclosures with or without front panel adjustable current controls are nice to
own because they can be adapted to a variety of HeNe tubes. However, this
definitely comes at a significant cost premium (unless you find a really good
deal) and are thus not usually a realistic option. They are unnecessary in
any case unless you expect to be playing with a variety of HeNe lasers and
need this compatibility. In fact, less expensive name-brand bricks may be more
robust!

However, most of the common 'lab' style HeNe laser power supplies are actually
just a standard power supply brick in a box with AC power cord, fuse, line
filter, on/off/key switch, power-on light, Alden connector to attach the HeNe
laser head, and an interlock connector and/or line voltage select switch on
some models. These units are, of course, somewhat more expensive than the
bare bricks and are also much less likely to show up on the surplus market.

See the chapter: Laser and Parts Sources for
possible commercial and non-commercial suppliers of HeNe laser tubes, heads,
and power supplies as well as things to watch out for when purchasing items
like this from private individuals or commercial suppliers other than major
laser companies. Manufacturers of new HeNe laser power supply bricks include
Laser Drive and Melles Griot; surplus sources include Meredith Instruments,
Midwest Laser Products, and MWK Laser Products. There are many others.

While a linear HeNe laser power supply driving a healthy compatible laser
tube should generate little or no RFI, it's truly amazing how much can
originate from a brick due to the switching frequency and its harmonics.
The biggest source is probably the inverter transformer, which if unshielded
can actually light up a neon indicator (e.g., NE2) placed on top of the
brick. Where RFI must be minimized, HeNe PSU bricks are often wrapped in
copper foil or mounted in shielded compartments. For example, when HeNe
lasers were used in hand-held barcode scanners, the bricks were usually foil
covered due to their proximity to sensitive electronics. Newer Hewlet Packard
and Agilent metrology lasers place an aluminum plate between the brick and
the laser tube. Some Zygo metrology lasers mount the PSU on an aluminum
plate and also add bypass capacitors on the input leads to suppress
transmitted RFI from interfering with the controller.

RFI can also originate from laser tube incompatibility, a tube that is
high mileage and/or near end-of-life, or incorrect ballast
resistor(s) resulting in plasma oscillations or other instabilities.

Assuming you know the power requirements of your HeNe tube or laser head, use
the following guidelines when selecting a brick type power supply:

The input voltage and current should be something available. :)

AC-input power supplies will need to be configured properly. Where
the voltage doesn't quite match your electrical supply as with power
supplies rated for 100 VAC instead of 115 VAC, there are two options:
(1) hope it survives on the slightly incorrect voltage or (2) add a
buck/boost transformer to adjust the line voltage. For the example
above, this can be a small 12 or 15 VRMS transformer with its input
connected across the line and its output connected in series with
the power supply input - with the proper polarity! For line voltages
that are near their nominal voltage, there is probably no problem as
the supply will have been designed to handle a +/-5 or +/-10 percent
input voltage variation, so an added few percent will be tolerated.
But where, for example, your line voltage is running very high like
135 VAC, connecting a 100 VAC brick directly may be risky.

DC-input power supplies usually have an acceptable range of voltages.
For example, a supply rated 12 VDC will usually - but not always - operate
correctly on 10 to 14 VDC. Too high or too low and it may be destroyed.
A regulated DC power supply of the correct voltage and adequate current is
best, but where that input range is fairly wide, an unregulated supply
can be used.

The operating current should be within the current range of the power
supply. If the supply has a fixed current setting, as long as this is
within about 10 percent, it will probably be acceptable. I would prefer
the error to be on the low side if there is a choice to maximize tube life
Laser power output may be slightly lower than its rating but probably not by
anything significant. However, some laser tubes or heads won't stay lit
at a current much lower than the optimum and a flickering laser is worse
than no laser as that can damage both the laser tube and power supply.

Where the current is adjustable, the voltage compliance range spec for
the power supply (printed on the label or elsewhere) often applies even
at max current. However, this is not always the case. So a supply with
a label value of 5.0 mA may have a reduced maximum voltage if set to 6.5 mA
even if the trimpot has enough current range. The current will then top
out at some lower value like 5.7 mA, but is not being regulated beyond
that point. The opposite could happen if reducing current (though this
may be less likely).

The operating voltage should be within the voltage range of the power
supply. The operating voltage is either specified explicitly for a laser
head or calculated from the HeNe tube voltage + ballast resistor voltage
drop (Io * Rb). Where only a single voltage is listed for a power supply,
assume it is the upper limit; the supply will probably operate reliably
between this value and 25 to 30 percent less (somewhat dependent on the
current as well). So, a supply listed as 2,450 VDC would be happy if your
laser head only required 1,800 VDC but don't go much lower unless you have
the actual specs and they indicate it is permitted. Some have a much wider
range, but finding out the hard way that your model doesn't isn't fun. Your
HeNe tube could end up with more current than it wants but more likely, the
power supply would become as dead as a - brick. :(

The electrical output power (operating voltage times operating
current) into the laser head or ballast+laser tube should not
exceed the rating of the supply. This in fact may be the most
relevant parameter for commercial HeNe laser power supplies.
In some cases, for a given operating current, calculating the
operating voltage based on the maximum output power rating of the
supply may be acceptable even if the resulting operating voltage
exceeds the value shown on the label or listed elsewhere. However,
input and output regulation will be worse near the ends of this
range. But for many applications, this may not matter.

For example, one particular sample listed 3.2 to 3.8 mA (adjustable
with a trimpot), 900 to 1,600 V, 7 W max. While one might think that
the 1,600 V is a maximum safe (for the supply) voltage to use, if the
operating voltage is computed from the maximum power (7 W) and operating
current (say 3.5 mA), then it is actually a much higher 2,000 V and
this is acceptable, subject to the comment about regulation, above.

However, unless the listing on the label explicitly says something like
"XX W max", it may simply be the power at the listed current and voltage.
Or, it may even be the input power! In short, your mileage may vary in
interpreting these specifications.

And, the power supply companies will generally protect themselves with
a statement something like: "The power supply should not be run under
conditions outside of any listed on the label as this may impact
regulation, stability, and life."

Generally, the starting voltage provided by a brick supply will be
adequate as long as the operating voltage is within its compliance range.
However, some hard-to-start HeNe tubes may benefit from running near the
lower-end of a larger power supply (with its higher starting voltage) as
opposed to near the upper-end of a smaller one.

Selecting a lower rated power supply as long as it meets the requirements,
above, will also likely be cheaper and a larger power supply won't get you
even one additional photon of laser output!

Of course other things like control inputs, CDRH delay, output connector
type, and size and weight, must also be compatible.

For high power (e.g., 25 mW and up) external cavity lab style HeNe lasers,
options are somewhat limited. Very few companies still manufacture
such lasers and they may actually use (possibly custom) models from
the laser power supply companies (see the next section) so it's worth
checking those when searching for a replacement.

Having said all that, based on analysis of a variety of HeNe laser power
supply bricks, it is becoming more and more obvious that while there may
be dozens of different models, there may only be a very small number
of truly different designs or even designs with different part values.
So, while a supply may have a maximum voltage listed on the label of, say,
1,600 V, it may indeed be perfectly happy operating at 2,500 V as long
as the power dissipation specification isn't exceeded, if that. So, in
the end, your mileage may vary, but if a supply runs stably with your
laser head, it may be fine regardless of what the label says! ;-)

Here are some of the common Melles Griot HeNe power supplies in recent,
if not current, production. Note that the power supply modules ("bricks")
are actually probably manufactured by Laser Drive, Inc. They are all
listed in terms of the output voltage compliance range. Differences
may still be present in terms of input voltage(s), output current,
wire termination, and physical size. Where part of a complete system,
the 05 may be replaced with 25.

The voltage range for some of these entries may be smaller
than actually provided by some of these power supplies, possibly
to account for variation in supplier specifications.
For example, the voltage range of the 05-LPM-379 which is listed here as
1,400 to 1,600 V, may be shown on the actual brick as 1,150 to
1,700 V. Since most Melles Griot power supplies have complete
specifications printed on a label, the information below would really
only need to be used as a selection guide, or where a model number (without
specs) is all that's available (as in an eBay auction).

Should only a single voltage be listed on the unit, the compatible range
*probably* extends somewhat above and below this value but there is no way
to be sure and it is probably not a good idea to push your luck too much on
the high side at least. Laser Drive generally spec's it to be +/-10
percent.

Many of the bricks have a trimpot for adjusting the current over a relatively
wide range even if only a single value for current (e.g., 6.5 mA) is printed
on the label. For example, an 05-LPM-379 (or 05-LPL-379) which may be
used with the 05-LHR-901 laser head (nominal current of 4.5 mA), typically
has an adjustment range of around 3 to 6 mA or more. However, some bricks
have no trimpot even in lab-style supplies. Unless an adjustment range
is shown on the label, the only way to know for sure is to locate the trimpot
(or a calibration seal covering it), usually near the input wires. A very
few (well I only know of exactly one - the 05-LPM-949!) have two trimposts
with the second one being to set the startup (CDRH) delay. (This may have
been a special OEM model used as a lamp igniter in a video projector. What
a waste!!)

AC line powered models:

All models start with 05-LPM for the brick or 05-LPL for those where a
"lab-style" unit is currently available (as of 2007, indicated with
a "*"). These have a brick mounted in a plastic box with power switch,
light, and fuse. Where part of a complete system, the 05 may be replaced
with 25. Higher voltage models are almost all 115/230 VAC compatible. But
some of the lower voltage units run on 115 AC only. (They saved the cost
of a wire.) The voltage tolerance is usually +/-10%. There are some
models that are spec'd for 100/200 VAC +/-10%. Whether these would
work reliably (without burning up) at higher line voltage (which may be
as much as 125/250 VAC, well outside the 10% tolerance) is questionable.

The following information was provided courtesy of Laser Drive (now part of
Martek Power) and applies to most
of their standard HeNe laser power supplies. For custom requirements, contact
Laser Drive directly. For each supply series, these show the available
minimum and maximum input voltage, the minimum and
maximum output voltage and current, as well as the *approximate* HeNe laser
output power for compatible laser tubes/heads. However, any single model
will generally NOT cover the entire range. More below.

Input voltage and current: This option determines the allowable
input voltage (AC or DC as appropriate). There may be some overlap.
Where there is only a single listed current, it is the worst case for
the lowest voltage option at maximum rated output.

Output voltage and current: Each series power supply may come in
several "Ranges", which determine the output capabilities of the specific
model supply. For example, with
the 101T, Range 2 specifies 1,500 to 1,800 VDC AND 4.5 to 5.5 mA. Note that
where only a single set of values is printed on the label (e.g., 1,600 V,
5.0 mA for the 101T), this uniquely determines that it is a Range 2 supply.
If a adjustment pot is present (possibly covered by a calibration sticker),
the allowable min and max current is determined by the Range of the supply
- 4.5 to 5.5 mA for the 101T.

Adjustment and compliance limits: Any given power supply
may operate outside of these specifications (current
and/or voltage), but could fail without warning due to overstress of internal
components. And this of course voids the warranty!

Starting voltage: These are approximate and will also depend on
the specific model and input voltage.

There were both AC and DC input types.
The following are all lab-style power supplies consisting of an AC-input
switchmode brick mounted in an oversize plastic case at a greatly
inflated price (unless we're talking eBay!).

All of these will also run the corresponding polarized "-PC" laser heads.

Really old Hughes lab power supplies are in a more-or-less
cubical metal box and use a high voltage transformer with a potted
high voltage module housing the rectifier/doubler/filter and linear
regulator.

Some have a current adjustment pot well hidden inside. It
is at the bottom of a recessed hole on the side of the potted HV section
against the sheet metal divider. So, 4 screws need to be removed to gain
access. These will drive somewhat higher power heads that what's listed
but your mileage may vary, and even those with identical model numbers
may differ slightly in voltage compliance. Some (like the 3509H) have
a glass tube in a socket. This is a thermal relay for the CDRH delay.
It can come loose in shipping and then the power supply will appear to be dead.

The only failures I've seen with these power supplies is that if severely
overstressed, the regulator inside the potted module may fail shorted and
the current will then be way too high. But the power supply can still be used
on a Variac to control current (but with some ripple in the output current).
Even if the regulator is functional, they can still be used at lower current
on a Variac.

The DC-input supplies were not as common as those with line voltage input.
The 3595H is one example with a nominal 12 VDC input for the intended
laser tube, probably for a barcode scanner. But since there is
no regulation, it will happily work over a wide range of input voltages
to drive tubes up to at least 5 mW (rated).

The duration wasn't very long so it's not known how these would hold up
in continuous service for the higher power tubes. There is an aluminum
block to which the chopper transistors attached that could be attached
to a heat-sink. Even without one, it didn't really get hot even with
the 05-LHR-120 tube at 6.5 mA. An aluminum plate (no fins) bolted to
the plate did get noticeably warm after a minute or so with the 05-LHR-150
run at 6.5 mA but not dramaticly so. For some reason, this supply would
not run an Aerotech OEM5R, which is similar to the 05-LHR-150. The tube
glowed throughout its gas volume but the discharge never struck down the bore.
All these tests used an 81K ohm ballast.

The supply seemed much happier driving the 05-LHR-150 with relatively little
heating of the aluminum plate. It would probably run continuously with
no problem. However, all these currents are close to the minimum at which the
tubes would stay lit with the 12K ohm external ballast. For example, with
81K ohms, the 05-LHR-150 would be stable down to below 4 mA. So, a compromise
might be required with perhaps a 35K ohm external ballast to force a lower
dropout current.

Finally, here is a really old Hughes supply - the 3582H. This is a large
linear power supply which includes adjustable current and internal adjustable
10 kHz modulation. Here are some photos:

Hughes 3582H HeNe Laser Power Supply shows the
overall unit. The Current and voltage capability are not known, though
based on the main filter capacitors, the voltage cannot be greater than
1800 V (4 x 450 V), and probably somewhat less. The meter goes to 10 mA,
but that may simply have been a convenient one to use. :) More below.

Hughes 3582H Interior shows what's on the
top of the chassis - not much. ;) The large can is a SOLA constant
voltage transformer. This presumably provides the AC line regulation.
The small transformer is for the HeNe laser tube high voltage. The brown
object is the starter pulse transformer. The HV DC and starting pulse
feed through the two HV diodes going to the laser head.

Hughes 3582H PCB shows most of the remaining
circuitry. A voltage doubler consisting of the HV transformer above the
chassis and a pair of rectifiers feed the 4 large capacitors, the main
filter. The four large 15k resistors are the internal ballast. The other
parts are related to the current regulation and modulation.

The actual ratings of the 3582H are not known. Assuming that the sample
I have is operating correctly, it would appear to have a very limited
capability in terms of the size/power of the laser. However, it may
have assumed a much lower ballast in the head than the ones I am using.
The very old two-Brewster Hughes laser heads like the 3184H had a very
low internal ballast of around 30K ohms. (See the section:
The Ancient Hughes HeNe Laser Head)
Even very low power heads with
the typical 75K ohm internal ballast would only pulse unless several
of the 15K ohm ballast resistors in the supply were bypassed. Even for
a 1 mW laser, all 4 resistors had to be bypassed for the laser to start
reliably and run with a acceptable current of 4 mA. A 0.5 mW laser
would then run at up to 5 mA, but could be set lower.

The modulation is interesting. Originally, it was thought that there would
be a signal input for modulation but that does not exist. The modulation
is fixed at about 10 kHz with an amplitude/depth range adjustable from 0
to enough to cause the tube to drop out. There are a pair of unlabeled
trim-pots that may perhaps set these limits.

So the current speculation is that the 3582H was intended for one of the
lower power 2-B heads, perhaps the 0.5 to 1 mW 3178H but probably not
the higher power 3184H.

Note: Not all of the HeNe laser power supplies in the next few sections
are "bricks" in the strict sense of the term (portions aren't potted!),
and the wiring may be via connectors instead of color coded wiring but
so be it. :)

Most of the HeNe laser power supply bricks you are likely to encounter will
have been manufactured by one of the following companies (in alphabetical
order):

Aerotech. Aerotech
is no longer in the HeNe laser business. Their product line was acquired
by Melles Griot (see below), who may have specifications and wiring info
(via email) for older Aerotech power supplies.

Laser Drive. Laser Drive
claims to be the largest manufacturer of HeNe laser power supplies on the
planet. Their Web site has detailed specifications for at least their
current power supply models in the PDF file
Connecting HeNe Laser
Power Supplies. They produce OEM HeNe power supplies for Melles Griot,
Coherent, and probably others.

Melles Griot.
Melles Griot has acquired the HeNe laser product lines of
Aerotech and Hughes (among others) and may still manufacture some of their
models (of lasers at least) for compatibility with older equipment. However,
their own HeNe laser power supply product line is quite extensive and
includes models for nearly every size HeNe tube or laser head. Most, if
not all, of these are actually manufactured by Laser Drive, above.

Power Technology.
Power Technology claims to be the oldest consistent manufacturer of power
supplies for HeNe lasers, in the business since 1969. Complete specifications
and wiring info available on their Web site.

Spectral.it. Spectral has only
one model but it is suitable for high power HeNe lasers requiring
between 6 and 16 mA at 2 to 4.8 kV.

Voltage Multipliers, Inc..
This company has a variety of high voltage power supplies and components as
well as HeNe laser power supply bricks.

Voltex, Inc.. Voltex is perhaps
the least well known of the major HeNe laser power supply manufacturers
but that is their main business. They now have universal supplies that
can run all but the largest HeNe lasers with only a current adjustment,
2 to 11 mA, 600 to 4,000 V. And a universal HeNe laser test set for
powering virtually all HeNe lasers which includes current
adjust, and voltage and current monitoring.

Others may simply be relabeled products of these major manufacturers but there
are also occasional units from lesser known companies, or those who would not
be expected to make any sort of power supply!

The laser head wiring will usually be done in one of the following ways:

In most cases, the outputs are either fat red and fat or thin black wires
for the ballast resistor/anode and cathode respectively with or without a two
prong female Alden Connector. The short prong is HV
positive and the long prong is HV negative, which is usually connected inside
the power supply to the negative input or chassis ground.

High power HeNe lasers may us something somewhat similar but with 3
prongs (the third one is for chassis ground which may be different than HV
negative). CAUTION: Siemens/LASOS and Spectra-Physics pinouts are NOT
the same!

Some Spectra-Physics lasers use a special 3 pin round connector (view
is looking toward power supply):

O Positive (Anode)
1
GND O 3 2 O Negative (Cathode)
o Interlock Prong

The GND may not actually be present on some power supplies. In most cases, it
is already connected to the negative elsewhere. The interlock prong
activates a microswitch in the power supply to complete the primary-side
circuit only if the power supply and laser head are securely attached.
This provides protection for the power supply but isn't present on all
models.

IMPORTANT: The design of these connectors is somewhat marginal for the
peak starting voltage that may be present, especiall with hard-to-start
tubes. Make sure the connector is fully seated with the locking
ring tight. Otherwise, it's possible for arcing to occur which will
damage the power supply connector and possibly the cable connector as
well by carbonizing the plastic between the pins.

Some Coherent, Jodon, Spectra-Physics, and other HeNe lasers use
a special high voltage BNC connector. They appear to be the 10 kV
series from Kings Electronics.
Since these are expensive ($25 to $35 each and probably special order from
an electronics distributor), and correctly assembling BNC plugs is
somewhat of an art form not easily mastered, replacing them with common Alden
connectors is often the best solution where one is damaged, or it is
desired to mate a laser head with a power supply that doesn't quite match.
Or, buy a dead laser head cheap on eBay and just use its cable! Of course,
sometimes these turn out to be fully functional. Don't you just hate it
when that happens? :)

Some power supplies may use spring loaded contacts if designed to mount
directly in a laser head (and probably have a built-in ballast resistor as
well) and others may just have push-on or other type connectors.

Read the label! Many supplies have detailed wiring info printed somewhere
on the brick.

If one wire is red and the other is a different color, the red one is
almost certainly the positive.

If there is a thick and thin wire, the thick one is almost certainly
the positive.

Measure the continuity of each output wire to Earth ground (AC supplies,
green or green yellow) or negative input (DC supplies, usually black).
The one with the lower resistance will be negative. This resistance will
most often be close to zero ohms and the other will almost certainly
measure open.

Use a voltmeter with a high voltage probe to test the output voltage,
but don't run the power supply open-circuit any longer than necessary.

Take a guess and power your HeNe laser head or tube with a mA meter
in the cathode return circuit. Turn it on just long enough for it to start
and take note of whether the polarity of the current is correct. Reverse
if necessary. CAUTION: DO NOT let the tube run for more than a few seconds if
the polarity is wrong! A neon lamp can be used in place of the mA meter -
only the negative electrode will glow. WARNING: Make sure the meter is
well insulated and don't touch it with power applied. If you guessed wrong,
the full output voltage will be on it! Take care when changing connections
as these power supplies can store a painful charge in their output capacitors.

Do the same but with a stack of 100K ohm resistors large enough to result
in approximately the operating current at the nominal operating voltage.

I do NOT recommend doing this with only the mA meter or neon lamp as a
load. While the manufacturer may claim short circuit protection, don't
believe it! Some power supplies will literally explode.

The CDRH (Center For Devices And Radiologic Health) of the Food and Drug
Administration delay mentioned with respect to some of these power supplies
prevents the beam from coming on for 3 to 5 seconds after power is applied
(i.e., should the ON switch be hit accidentally) and may be needed to meet
certain regulatory requirements.

Whether or not each of the various control inputs or status outputs are
present on a particular unit is model dependent.

The following two sections have a summary of some of the wiring color codes
I have come across for HeNe laser power supply bricks.

The following run off of the AC line. Usually, they can be wired for either
115 VAC or 230 VAC operation but sometimes the manufacturer saved a few cents
and doesn't provide this option. There is no standard color coding for these
bricks though the examples below cover most common models. Earth ground
(green or green/yellow) should be tied to power line earth ground for safety.
The only place the ground wire goes inside the brick is usually the HV
return - everything connected to the AC line is isolated. It's best is to make
this a direct connection but it can also be via a 100 ohm resistor to
avoid ground loops. Unless specifically noted as not necessary by the
manufacturer, a fuse should be installed in series with the Hot wire. A
rating of 0.5 to 1 A should be adequate for most small bricks but a several
amp fuse may be required for those driving higher power lasers. The fuse
is to deal with catastrophic internal failure like a shorted rectifier which
could cause parts to explode if there is no protection. A momentary output
fault shouldn't blow the fuse. Therefore, the fuse can be soldered
in permanently since it should never have to be replaced. See the section:
Importance of Fusing Power Supply Bricks.

If you're curious as to how the dual voltage capability is done, here is the
simplified circuit for the AC line front-end of one of these power supplies:

For operation on 115 VAC, power is applied between L1 or L2 (which can
be tied together but this isn't necessary) and L3. D1/D3 and
D3/D4, along with C1 and C2, act as a voltage doubler which provides
approximately twice the peak line voltage (about 320 VDC for a 115 VAC input)
to the chopper.

For operation on 230 VAC, power is applied between L1 and L2, L3 is left
open. D1/D2/D3/D4 then form a bridge rectifier which provides approximately
the peak line voltage (about 320 VDC for a 230 VAC input) to the chopper.

In either case, the polarity of the AC wiring (e.g., for 115 VAC whether Hot
goes to L1/L2 and Neutral goes to L3 or vice-versa) shouldn't matter in terms
of safety or performance. But follow the manufacturer's recommendations if
they show a particular wiring arrangement.

It should be possible to confirm the correct wiring for your line voltage if
the power supply isn't labeled or listed below ASSUMING YOU ARE SURE THAT THE
WIRES YOU ARE DEALING WITH ARE FOR THE AC INPUT (you really don't want to be
pumping line voltage into a logic level enable!). Identify the safety
(Earth ground) wire: It should be green or green with a yellow stripe,
but age and use might fade it to yellow. Check resistance to the fat
black HV output wire - only the ground wire should show low ohms.
Usually they are tied together internally since grounding the laser head
is really its only purpose on a potted brick. Then use a Variac limited to a
maximum voltage of around 120 VAC to feed the supply with its output connected
to a known good compatible laser head or HeNe laser tube with 75K ohm ballast
resistor. Try each of the 3 possible pairs in turn. For 2 of these, the
laser should start at about 60 to 85 VAC. The 3rd will be the 230 VAC
configuration and may not start at all on your maximum of 120 VAC. However,
locating this pair is a good way to confirm which wires should be tied
together for 115 VAC operation. Note that assuming you have the correct
three (3) wires, there is no combination (even tying together L1 and L3 or L2
and L3) that will produce smoke from a 115 VAC input and every one but
using only the 230 VAC pair alone will result in correct operation. (I just
consider it cleaner to tie L1 to L2 as opposed to leaving one of these open or
tying it to L3.) CAUTION: There are just guidelines - your mileage may vary!

CAUTION: All unused/unconnected wires should be insulated.

WARNING: The "CDRH loop" or "CDRH delay" on many if not most or all AC-input
bricks is NOT isolated from the AC line. So, attempting to repurpose it as a
logic enable could result in fireworks or worse unless some sort of
opto-coupler is used. This additional danger is not mentioned in any
HeNe laser power supply instruction manual I've ever seen. :( :)

Here are the color codes for some common AC line powered models:

Aerotech (115/230 VAC type). Red (or possibly other color except
green or green/yellow) wire loop enables CDRH delay (cut to disable). The
AC input wire color may depend on model. Green or green with a yellow
stripe is earth ground and connected to cathode lead internally. (Some
typical specs: LSS 05 - 1,400 V +/-200 V, 4.0 mA; LSS 2 - 1,900 V +/-250 V,
5.0 mA; LSS 5 - 2,500 V +/-300 V, 6.0 mA, LSS 10L - 3,000 V +/-300 V, 6.5 mA.
Current may actually be slightly
different depending on option but will probably be marked if not equal to
what's shown above. There is no current adjust pot accessible from the
outside. I know exactly where the pot is (by examining a unit that was
partially destroyed) but that doesn't help since the rock-hard Epoxy potting
compound surrounds it and drilling a hole at that location would be useless.
(However, it might be possible to drill into the pot, discombobulating it,
but providing access to its connections. This is left as an exercise for
the student.) Note that the designation 05, 2, or 5 means that the supply
is sort of intended for 0.5 mW, 2 mW, 5 mW, or 10 mW HeNe lasers,
respectively. But of course this isn't etched on stone tablets!
The same color code is used for rectangular bricks and integral
cylindrical power supplies.

115 VAC: Brown to black, blue (or gray) open.

230 VAC: Blue (or gray) to brown, black open.

Coherent (115 VAC types). Green is earth ground but may NOT be
connected to cathode lead internally. Purple wire loop enables CDRH delay
(cut to disable).

115 VAC: Whites (tied together) to yellow.

230 VAC: White to white, yellow open.

Current select: Connecting white/orange to white/purple selects 6.5 mA;
open selects 5.25 mA (model CR90-115). Other Coherent models may
differ. A switch can be used in place of the jumper plug. A pot of
around 25K ohms can be used to vary the current continuously from
about 5.25 mA (25K) to 6.5 mA (0). A reverse audio taper (to get a
more uniform change with respect to angle) with a switch at the far
CCW position (to get the 5.25 mA) may be best. Or, a 6 position
switch with resistors to select the current can be used. Then, no
mA meter is needed - just count clicks! :) The appoximate resistor
values for mine are: open (5.25 mA), 24K (5.5 mA), 10K (5.75 mA),
3.9K (6.0 mA), 1.2K (6.25 mA), and 0 (6.5 mA), give or take 0.1 mA. If
using a "break-before-make" type switch (most common), put a 22 uF (10 V or
higher) capacitor between the white/purple (+) and white/orange wires (-)
to minimize the current dip during
switching. But note that the current will start at 6.5 mA and then
decay to the set current in a fraction of a second at power up
as the capacitor charges. This is generally of no consequence.
With a "make-before-break" type switch, wire as a tapped resistor
instead of separate resistors and there will be no glitch. Details
are left as an exercise for the student.

Laser Drive (115/230 VAC type). Violet (or possibly other color except
green or green/yellow) wire loop enables CDRH delay (cut to disable). Green
or green with a yellow stripe is earth ground and connected to cathode lead
internally. There may be a current adjust pot.

115 VAC: Whites (tied together) to yellow.

230 VAC: White to white, yellow open.

TTL high (+3 to +5 VDC) required between yellow/white (+) and
yellow/black (-) to enable laser.

Some 115 VAC Laser Drive power supplies may only have two whites (or other
same color) for input. In this case, 115 VAC goes between them. There are
no other options. :)

Melles Griot (115/230 VAC type). Violet wire loop enables CDRH
delay (cut to disable). Green is earth ground and connected to cathode
lead internally. There is usually a current adjust pot near where the
input wires exit the brick (may be covered with tape). Some models may not
have both 115 VAC and 230 VAC options and/or the CDRH loop. (Some or all
Melles Griot power supplies are manufactured by Laser Drive.) An additional
number after the 3 digit model may be present to indicate the factory
default current setting (changing it voids the warranty, I'm sure you care!)
or a code for the maximum power rating of the supply. (Melles Griot power
supplies are manufactured by Laser Drive, so check there (above) if your
power supply has a different color scheme than the following.

The following is for their model 05-LPM-379:

115 VAC: Whites (tied together) to yellow.

230 VAC: White to white, yellow open.

This is for models like the 05-LPM-379, 05-LPM-340, 05-LPM-903, 05-LPM-915,
and 05-LPM-939:

115 VAC: Brown and blue (tied together) to white,

230 VAC: Brown to blue, white open.

Power Technology (115/230 VAC type). Violet wire loop enables CDRH
delay (cut to disable). Whether AC inputs are white or gray depends on
model. Green or green with a yellow stripe is earth ground and connected to
cathode lead internally.

115 VAC: Whites or grays (tied together) to yellow.

230 VAC: White to white (or gray to gray), yellow open.

TTL enable: Separate pair of wires, on of which is usually White/black.
Requires +5 VDC on white/red, high to enable, low to inhibit. These color
codes and operation vary by model. The actual voltage may not matter
very much with anything from 3 to 10 VDC turning on the laser. Other
possible colors include white/yellow and yellow/white.

The following operate from a low voltage DC input. Most require 12 VDC
but some may use a lower voltage (like 6 VDC) or higher voltage (like 18 or
24 VDC). Where covered with a metal foil shield, this will also be connected
to the cathode. Red, violet, or other color (except green or green/yellow)
wire loop may be present to enable CDRH delay (cut to disable). Regulated
input voltage may be needed depending on model. Positive input is almost
always red; negative will likely be black or blue. (Both of these may be
of thicker gauge than the other low voltage wires.) There is usually a
fairly wide range of acceptable input voltage for any given type. For
example, those rated at 12 VDC will generally be perfectly happy on anything
from 10 to 14 VDC. The input may not need to be regulated or ripple-free
as long as it stays within that range.

A general idea of the proper voltage for such a supply if not labeled or
listed below can be found by driving it from a variable DC source with a
known good compatible laser head or HeNe laser tube and 75K ohm ballast
resistor connected to its output. Increase the input voltage until the laser
starts but don't go above 12 V even if nothing happens until you have checked
for the possibility of a logic signal being needed to enable operation.
Figure the voltage at which the laser starts to be around 60 to 75 percent of
nominal. If the thing is still, well, dead as a brick at 12 V, it
might require a higher voltage but even a 24 V unit should probably
show some signs of life (audible whine or ticking) on an input as low as 12 V
if it is enabled and functioning correctly. Another indication that the
supply is doing something is substantial input current - at least 100 mA even
if the laser hasn't started. However, if it looks like a near short circuit,
you might have the polarity reversed (some models have a diode across the
input to guard against this possibility)! Others may be damaged or killed
by reverse polarity. CAUTION: These are just guidelines - your mileage may
vary!

Here are the color codes for some common low voltage DC powered models:

Aerotech. There are several variations on input voltage range.
They also have a current adjustment pot with a range of around 3 to 5 mA
(on the low power versions, a higher range on larger ones.
The AC line powered Aerotech power supplies that I have do
not have pots accessible except perhaps with a chisel!). The factory default
current is typically 4 or 5 mA for the LSS 05 (0.5 mW) units and 5 mA for the
LSS 2 (2 mW) units. The ground return loop and operation monitor are not
both present on a given unit and may not be present at all. The enable may
not be present on all versions.

Aerotech power supplies designed to fit in the end of a cylindrical laser
head have a built-in ballast resistor, so as long as the connection to the
anode is no more than a couple of inches, nothing extra is needed.

+12 (minimum for stable output on this unit): Red.

Ground (common): Blue.

Ground return from cathode: Aluminum end-plate or green/yellow loop,
which jumpers high voltage return to DC input return and can be used to
monitor output current. CAUTION: Do NOT attempt to run the power supply
unless there is a low resistance connnection (like a mA meter) between
the two wires of this loop, as bad things happen and the power supply
may die.

Enable: Yellow - ground to turn on laser.

Coherent (these are probably made by Laser Drive):

+12 VDC: Red.

Ground (common): Black.

Enable: Yellow - ground to turn on laser.

Laser Drive model 103-23:

+21 to +31 VDC: Red.

Ground (common): Black.

Enable: Yellow - ground to turn on laser.

Laser Drive model 103-06:

+15 VDC: Red.

Ground (common): Black.

Enable: White with red stripe - tie to +15 VDC to turn on laser.

Power Technology model L23D-DW. This is really old, probably the
power supply for the Spectra-Physics 084 tube used in early '80s barcode
scanners.

+9 to +17 VDC: Red.

Ground (common) and laser tube cathode: Black.

Enable high: White with red stripe. This works using a 1K ohm resistor
to the positive input (red wire). I do not know if it is inteded to be TTL
compatible or whether it can be connected directly to the red wire.

Aux output: Thin white wire, probably -V for RS232 driver.

Yahata Electric Works, Ltd. model HVR-C234H-1. They call this a "High
Voltage Unit" but it's a HeNe laser power supply. Input is via a 3 pin
connector, not color coded wires, and only the high voltage circuitry is
potted. Output specs are: 2.35 kV at 6.5 mA connections are via 3/16" (+)
and 1/8" (-) FastOn lugs. CAUTION: This power supply is NOT short circuit
protected as I found out! :(

Trigger: Middle pin, pull to ground and release to start, leave open
to run, ground to stop.

12 VDC type, manufacturer unknown, Herbach and Rademan part number
TM91LSR1495. Output on 1/4" FastOn lugs with hand printed lables which
may be wrong - double check polarity before powering a laser!

+12 VDC: Red.

Ground (common): Black.

12 VDC type, models unknown (from barcode scanners):

+12 VDC: Red.

Ground (common): Black.

Enable: Yellow - ground to turn on laser.

And another:

+12 VDC: Red.

Ground (common): Black.

Enable: While/yellow - high (+4 V) to turn on laser.

Scan motor: Black/white, white/black, and blue/black.

Hand-held barcode scanner head brick, manufacturer unknown. This may be a
sort of Standard. (My sample may have come from a Symbol Technologies
LS-6000/6500 but it had no markings.) Output is fixed at 3.0 mA
for a Melles Griot 05-LHR-006, Siemens LGR-7651, or Uniphase 1007 6 inch tube.

+12 VDC: Red.

Ground (common): Black.

Enable: Yellow or white with yellow stripe - turns laser on when
connected to a level between +5 and +12 VDC. (For normal use, leave
attached to red wire.)

-Aux output (measured -13 VDC): White or white with black stripe.
This unregulated voltage is intended to power other circuitry, possibly
the RS232 driver used by the scanner.

The AUX voltages are unregulated and intended to power other circuitry,
probably an RS232 driver used by the scanner.

Partially potted supply from Spectra-Physics barcode scanner head. This
one is a strange shape, a tapered flattened cylinder, mostly copper foil
covered, but with some of the low voltage circuitry exposed. Output is
fixed at 3 mA for a Spectra-Physics 007 or Melles Griot 05-LHR-007 4-3/4"
tube. It uses a 7 pin header for most connections with pin 1 on the far
right facing the unit. OK, this isn't really color coding but so be it. :)

+6 VDC: Pins 6 and 7.

Ground (common): Pins 4 and 5.

Enable low: Pin 3.

Enable high: Pin 2 (must be current limited).

+12 VDC regulated output: Pin 1.

There are several ways to control the power supply:

Connect pin 2 to pin 3: The laser will be on as long as +6 VDC power is
applied.

Connect pin 2 through a 1K ohm resistor to +6 VDC. Connect pin 3
to Ground (common). The laser will be on as long as +6 VDC power is applied.

Connect pin 2 through a 1K ohm resistor to +6 VDC. The laser will turn on
ONLY when pin 3 is grounded.

CAUTION: The resistor between pin 2 and +6 VDC is required to prevent
smoke. Do not omit!

I haven't confirmed it but believe pins 2 and 3 are compatible with TTL
levels. The 1K ohm resistor will still be required for the signal
driving pin 2. Pullups may also be needed to achieve high enough HIGHs.

Do NOT be tempted to cut corners and omit safety devices like fuses when
wiring up a HeNe laser power supply brick! Well, any power supply, for that
matter, but because these are totally potted, there are added dangers. Even
well designed commercial power supply bricks may lack internal protection and
have been known to explode like grenades if not wired up with a fuse or the
wrong fuse is used. A proper fuse will prevent or at least greatly reduce the
possibility of a component cooking for long enough to cause this sort of
damage. Many or even most newer brick power supplies do have an internal fuse
which will blow should there be a failure of a major component, but not an
overload like a momentary short on the output. (Of course, once blown, the
power supply is only good as a rectangular hockey puck.) Unless
the specifications or label states: "Internally Fused" or
something similar, add one of your own in the Hot side of the AC line. If
you acquire a power supply already wired (e.g., on eBay), check to see if
there is a fuse and add one if not. It's cheap insurance.

Electrolytic capacitors are probably the components most susceptible to this
cranky behavior. Where out in the open, a blown capacitor (due to a shorted
line rectifier, for example) will just result in a POP, smelly gas, white
smoke, and a whole bunch of capacitor guts (foil, etc.) all over everything
(though such capacitors have also been known to just vanish into thin air).
However, when totally encased in solid Epoxy, there is no place for the gas to
go. So, pressure builds up, probably to several hundred PSI, before something
lets loose. This is a rare occurrance but one you don't really want to
experience!

I've recently come across a defective Melles Griot (made by Laser Drive)
05-LPM-370 power supply brick that was on its way to exploding with a
noticeable bulge in the top surface. This was scary as that implied
some significant pressure inside. It was being powered by a Variac (5 A
fuse) into a stack of resistors (about 500K) and a 5 mW tube in an attempt
to make it maintain a stable discharge (the tube was flashing). I don't know
if a proper fuse would have blown though. :(

An old Aerotech brick (unmarked, but one of the large long ones that typically
run 10+ mW lasers) also may have developed a bulge while running a laser
head that should have been compatible. While the bulge wasn't that severe,
was only modestly warm to the touch, and *might* have been there all along,
that's not normal and I didn't wait to find out if the thing would explode. :)

Here are specific instructions for some common Laser Drive bricks:
314S - 1 A, 314T 115 VAC 60 Hz - 0.4 A, 314T 220 VAC 60 Hz - 0.2 A, 230 VAC,
50 Hz - 0.25 A (all slow blow type). For other types where the recommended
fuse rating isn't available, use 1/2 A for small power supplies (e.g., up to
5 mW lasers), 1 A for larger ones. Slow blow fuses really should be fine as
the sorts of failures being protected against are going to put many times the
rated current through the fuse. However, I've usually had no problems with
standard fuses. Since these fuses should not blow under normal operation and
even reasonable abuse, just solder them to the Hot power leads of the power
supply and line cord (but take care that the heat of the soldering iron
doesn't melt the fuse element!).

I had a very nice Laser Drive 380T that failed in this manner when it was
turned on with the output accidentally shorted. A 1 inch chunk of potting
material flew off as a result of two unidentified parts vaporizing inside.
They may have been fusable or other resistors in series with the output of
the voltage doubler at the front-end. I doubt that a fuse would have saved
the power supply, but it might have prevented the blast.

While a fuse won't guarantee that there will be no fireworks, it should
reduce the chances.

The following is what Laser Drive recommends for their 380T HeNe laser
power supply bricks, rated at 3800 V, 6.5 mA. (This was actually found
on a JDS Uniphase-labeled brick):

The Transient Voltage Suppressor or Varistor goes after the fuse
and limits the peak voltage in the event of a power surge. Even so, the
state that these modules should only be used inside another case. :)

Even though DC-input bricks run at lower voltages and typically won't
have the same amount of explosive energy available, a properly sized
fuse is also important, if for no other reason than to protect the
DC power supply feeding it! If there is no internal fuse, the typical
failure will put a nearly dead short across the input if the chopper
transistor fails. The label will often list the maximum current, so a
fuse rated at somewhat above that will be suitable. If the supply is being
used with a laser tube of much lower power than what is possible with
the power supply, a smaller fuse may suffice.

(From: Someone who would rather remain anonymous.)

Besides the obvious dangers of these little power supplies, such as the high
voltage at low current, there is another danger anyone powering up a HeNe
brick power supply should be aware of. This could save you a trip to the
hospital or worse - HeNe laser power supply - Proves the "Big Bang theory":

ALWAYS use a properly sized fuse in line with your power supply! Everyone
knows this, right? But did you know that there are some commercial HeNe laser
power supplies that can actually EXPLODE if there just happens to be an
internal problem in the supply or the laser head when powered up if a fuse is
not used in-line with the input, or if there is a fuse but it is grossly
oversized in it's amperage rating, or maybe even if it is of the correct
amperage but a "slow blow" type instead of a "fast blow"?

I have heard of these certain power supplies actually exploding within 1 to 2
seconds after being powered up, well today, it happened to ME while testing a
lot of power supplies for 2 to 3 mW HeNe laser heads! No names will be
mentioned, but the manufacturer's initials are L.D.

The power supply that did explode on me was fused, but someone, (no not me)
put in a 20 amp fuse instead of a 1.5 - 2 amp. Well this was a used surplus
unit pulled from medical equipment, of which I was testing out several. The
first 5 worked just fine, but the 6th one had other ideas. Little did I know
when I first applied the required 115 VAC that this was a bad power supply,
but that's why I was testing them.

Well, within 1.5 seconds of applying power to it, surprise, surprise - the
entire side of the plastic case and a lot of the hard Epoxy potting suddenly
became dangerous flying shrapnel. Plus, the resulting extremely loud BANG
was deafening and the resulting shock wave felt like being hit in your inner
ear by a speeding golf ball at the world tournament. I was only 4 feet away
from it at the time, but even now, 6.5 hours later, my ear is still quite
painful. I would liken the sound of the blast to a 12 gauge shotgun being
fired within two feet of your ears. The flying shrapnel could have caused
very serious injuries. But thankfully, it didn't hit any of us. (But, what
about the laser tube? :) --- sam.)

The catastrophic failure occurred because to large a fuse was used but I
guarantee that the results would have been NO different with this particular
power supply if no fuse was used.

What actually happened was that because of a malfunction in the power supply
(probably a shorted rectifier in the AC line input), a 3/4" x 1/2" electrolytic
capacitor exploded inside the case thus turning this power supply into a hard
Epoxy and plastic BOMB.

I do not believe that all commercially produced HeNe laser power supplies can
react this way if powered up with an internal problem. But trust me on this
one: Be sure to use a fuse and of the correct rating and type because you don't
want to find out the hard way.

An executive of the above unnamed manufacturer said that if a fuse would
eliminate this exploding problem when the power supply goes south. I also
noticed that this, now totally dead power supply drew much more current than
is normal from the 115 VAC input when first powered. This would have blown
a fuse of the correct size. However, I guess we will never know if this
actually would have prevented the power supply from exploding! :(

Most HeNe laser power supplies which use a DC input require a voltage between
9 to 12 VDC. However, some may require one that is higher like 18 VDC or
20 to 30 VDC (the latter typically originate from larger barcode scanners or
older HeNe laser based LaserDisc players). Wall adapters for these are not
that common. (Note that bricks using 115/230 VAC may also work with 300 VDC
if wired for 230 VAC since their input is a rectifier and filter but AC wall
outlets are generally more readily available!)

Here are several simple regulated power supplies which may be constructed
easily using parts available from Radio Shack, or possibly from your junk
box. (1) and (2) are basically the same but cover the 9 to 12 V and 20 to 28
V ranges, respectively. (3) is for the higher voltage input but may be built
using a 12 V transformer. However, if you have a suitable transformer, (2)
is preferred to (3). These can easily be modified for your specific
requirements (like 18 VDC). T1 can be an AC wall adapter with an adequate
current rating but realize that their output voltage can vary by a ratio of 2:1
depending on load (they use mediocre transformers!). A low voltage power
transformer will generally have a much stiffer output characteristic - less
change in output voltage versus load current.

It is always s good idea to have a fuse on the output, especially where the
supply is capable of much higher current than required for the brick. Where
T1 is an normal power transformer, a fuse should also be included in its
primary. If T1 is part of a wall adapter, this isn't required as it is
already protected internally.

The following assume a power requirement of 10 W max. Depending on the actual
ratings of your brick, component values may need to be changed.

Output 9 to 12 VDC:

This one uses a 12 V, 1 A power transformer
with a bridge rectifier, filter, and IC regulator. The diodes can be 1N4001s
or better, or a bridge rated at least 50 V and 1 A.

This one uses a 24 V, .5 A power transformer
with a bridge rectifier, filter, and IC regulator. The diodes can be 1N4002s
or better, or a bridge rated at least 100 V and 1 A. For a +15 to 21 VDC
output, use an 18 V power transformer and adjust the values of R2 and R3
appropriately.

This one uses a 12 V, 1 A power transformer
and is a voltage doubler feeding an IC regulator. The diodes can be 1N4002s
or better (at least 100 V, 1 A. It may even be possible to use the AC wall
adapter from an older modem for T1 (at least near the lower end of the
voltage range) but testing would be required as the actual performance
of these cheap transformers vary quite a bit. I tried a wall adapter from a
pre-Jurassic 2400 baud modem - that worked fine with the voltage set between
21 and 26 VDC powering a Laser Drive model 103-23 HeNe laser power supply
with a 1 mW (6 inch long) HeNe tube.

The LT1084 is a modern low dropout IC regulator but an LM317 can also be used.
(However, the voltage rating of T1 may need to be increased by 1 or 2 V to
compensate for the added voltage drop of the older technology regulator.)
Or, use a fixed positive regulator (e.g., 7812 or 7824) which would eliminate
the resistors. A 150 uF aluminum electrolytic capacitor can be used in place
of the tantalum (required for stability with the LT1084, only a few uF is
required for the LM317 or 78xx regulators). Fixed or adjustable negative
voltage regulators can also be substituted though there is no benefit unless
that's all you have in your junk box. Just reverse all the diodes and
capacitor polarities. A good heatsink should be put on the regulator
especially if the output voltage is much lower than the voltage on
the main filter cap(s).

CAUTION: Double check the pinout for your IC regulator - they are not all the
same. Here are some examples (all views from the front):

The following two sections describe the required connections and additional
circuitry that I used to make complete lasers using two types of HeNe tubes
and power supplies that were available from Herbach and Rademan. This will
be similar to what is required when wiring up any compatible combination of
HeNe laser power supply bricks and HeNe laser tubes or heads. (Note that I
am not necessarily recommending H&R as a surplus laser supplier - they
are quite overpriced in this area. However, I didn't know any better when
I purchased my first HeNe lasers!)

This uses a short (140 mm) HeNe tube (Siemens LGR7655 or equivalent) and
no-name power supply running off of 9 VDC. I mounted these in a case which
was from a 1/8" cartridge tape backup system in its former life. In order to
obtain regulated 9 VDC, an LM317 IC regulator on a heat sink was added along
with a power switch, power-on LED, and the required ballast resistor of 150K.
The ballast resistor was determined by monitoring the HeNe tube current and
selecting values until the current was correct (apparently, this particular
brick has no internal current regulation). The HeNe tube was mounted on
standoffs using a pair of nylon cable clamps and aimed through a hole drilled
in the plastic case which.

It may be easier to locate a 12 VAC, 1 A wall transformer since these were
commonly used to power older obsolete modems. In this case, add a bridge
rectifier and a 5,000 to 10,000 uF, 25 V filter capacitor to the input.

Using a 7809 or similar fixed 9 VDC positive voltage regulator in
place of the LM317 would save a few components. And the switch and
LED aren't exactly essential. However, unless the input power source
has an appropriately sized secondary fuse (e.g., 1 or 2 A, not 20 A),
the fuse should be included.

WARNING: Double check polarity before powering a laser. The unit I first
received had the HV polarity reversed from the hand printed labels. I sent if
back and they simply stuck new labels over the old ones and returned it. :)

HeNe tube: Siemens LGR7631A with attached ballast resistor and
Alden HV connector. Its specifications are similar to the 1.5 to 2.5 mW tubes
listed in the section: Typical HeNe Tube
Specifications.

Power supply: Laser Drive model 4009479.

This particular brick can be wired for either 115 VAC or 230 VAC operation and
includes an 'enable' input which must be pulled up to turn on the tube (thus
the 9 V battery visible in the photo). Likewise, rather than using a separate
power supply just for this, I provided a battery holder with 4 AA cells. Even
old tired decrepit ones work fine in this application! The only other parts I
added were the line cord, power switch, fuse, light, and enable switch.
Everything is mounted on a nice wooden frame painted flat black. :)

The ballast resistor was already built into the tube mounting so that this
was truly a 'plug-and-play' assembly.

BLACK and green wires are joined inside power supply so tube cathode
will be tied to earth ground when plugged into properly grounded outlet.

Wiring shown for 115 VAC. For operation on 230 VAC, use white wires on
each side of line and leave yellow wire unconnected.

A closeup of the components of another similar unit is shown in
Siemens LGR7631A HeNe Laser Tube and Laser Drive Power
Supply Brick (Components) (photo courtesy of: Effie Wiegand
(bestofme@home.com). Note the black 'button' on the right-end of the tube
(attached to the plastic anode insulator). This contains a slightly angled
glass plate presumably to protect the tube's output mirror. I found it to be
quite dirty on one sample I acquired - the beam quality was terrible until it
was removed (just pulls off). In fact, I believe the previous owner sold the
laser for next to nothing due to the messed up beam! These also came with a
diverging lens on an adjustable mount of sorts (visible in the lower right of
the photo). I assume this to be the first lens in a beam expander and
collimator (but the other components stay with whatever sort of barcode
scanner, printer, or other equipment from which this laser originated).

I installed an Aerotech LSS 5(6.5) HeNe laser power supply in an aluminum
Minibox(tm) chassis to create a 'lab' style unit capable of driving HeNe
laser heads requiring 2,200 to 2,800 V (including ballast resistor) at 6.5
mA. These are typically 4 to 12 mW red HeNe lasers (or lower power other color
HeNe lasers) though the specific requirements will depend on model.

This arrangement is basically what is inside most modern commercial 'lab'
style HeNe laser power supplies - a standard brick model mounted in a box
with AC power cord, fuse, line filter, on/off/key switch, power-on light,
Alden connector to attach the HeNe laser head, and an interlock connector
and/or line voltage select switch on some models - along with an inflated
price! :)

Two aspects of power supply operation should be checked as soon as possible
after powering up your HeNe tube for the first time: polarity and current.

If the power supply is a name brand unit or was pulled from a device like a
bar code scanner, it is safe to assume that the connections (the output in
particular) are correctly labeled. However, some power supplies sold to
hobbyists (e.g., from places like Herbach and
Rademan Co. or HSC Electronic Supply)
have been known to have incorrectly marked high voltage connections. (The
models I have seen this on are a low voltage inverter type with red and black
wire leads for the 9 or 12 V input and spade terminals for the high voltage
marked with hand printed or typed stickers for VA and VC - apparently at
random.) If your power supply looks like it was put together in someone's
basement, this should confirmed to prevent damage to the HeNe tube from
reverse polarity.

At the same time, the HeNe tube current can be checked.

Put a 1K, 1/4 W resistor in series with the cathode return and measure across
it with a voltmeter for the correct polarity and current. The end of the
resistor attached to the HeNe tube cathode should be positive and the current
will be 1 mA/V of your reading. Also see the section:
Making Measurements on HeNe Laser Power
Supplies.

So you have a couple of HeNe laser power supply bricks for use with 1 to 2 mW
HeNe lasers and someone just gave you a 5 mW HeNe laser head. Is it possible
to use the two in series or parallel instead of buying or building a suitable
power supply?

Larger HeNe tubes require much more operating and starting voltage, and
possibly more current as well. The current difference is of less consequence
than the voltage difference.

For example, a 5 mW tube may have a nominal current rating of 6.5 mA but will
probably run just fine on 4 or 5 mA (though a higher value ballast resistor
may be needed to maintain stability). However, the 5 mW tube will require
about twice the operating and starting voltage of a 1 mW tube (thus requiring
a series connection if such a kludge is possible at all).

Whether the power supplies can be hooked in series or parallel depends on
their design. The inverter types in particular (which includes all bricks),
could become mighty unhappy. I wouldn't recommend connecting HeNe power
supplies in series without knowing exactly what's inside. You could
end up with two dead power supplies and there could also be safety issues with
respect to insulation breakdown, explosive disassembly of internal components,
and other undesirable and unfortunate consequences.

However, it may be possible to connect them in parallel to increase the
current capability. Where both power supplies regulate using high-side
current sensing (as is common with bricks since the negative returns usually
go directly to ground), then this may be possible if both power supplies
are able to operate within their current compliance range. Additional
details are left as an exercise for the student. :)

And, there is one special case which does have a good chance of success
as described in the next section.

So, you got a great deal on a HeNe laser power supply rated 2,500 V at 6.0 mA.
This would be great for powering a 5 mW HeNe laser except for one problem: Most
5 mW HeNe lasers run optimally at 6.5 mA. Now, as a practical matter, 6.0 mA
is usually still just fine and most people would never notice the difference
in output power, stability, or (optical) noise. But, for the purist, 6.5 mA
would still be best.

It should be possible to add that extra 0.5 mA from a second (non-HeNe laser)
power supply in parallel with a HeNe laser power supply with a fixed output
current. This would usually be used with a potted power supply brick lacking
a current adjust pot since otherwise, it's usually a simple matter to just
modify it directly!

All that is needed is a power supply capable of providing the additional
current at a voltage greater than the operating voltage of the HeNe laser
(tube + ballast resistance). The source can be a potted high voltage
(voltage regulated) module, voltage multiplier from an isolated transformer,
or some other similar low current power supply. Feed the auxiliary power
supply into the top of the ballast resistance via a HV isolation diode (e.g.,
microwave oven or CRT HV rectifier) to prevent the starting voltage from
appearing on the auxiliary supply. To protect the HeNe laser power supply
brick, it won't hurt to install a similar HV isolation diode in series with
it's output as well (though this is probably not essential). The amount of
added current can be adjusted with resistors in series with the auxiliary
source. The negative outputs of both power supplies should be tied together.

Since the current regulator of the HeNe laser power supply doesn't see the
extra current, it won't fight the auxiliary power supply. However, it is
the responsibility of the auxiliary power supply to do the regulation of
the added current. Since it's only a very small current, series resistors
are probably good enough - no active regulation is needed. A current
regulated supply could also be used as long as the current sensing of both
the power supplies takes place separately. This usually means the auxiliary
power supply senses current in the HV feed, not the return (which is usually
grounded), since this is what most HeNe laser power supply bricks do.

I've tested this approach with an Aerotech LSS 5(L) AC line powered brick fixed
at 6.0 mA and a variable HV DC power supply. The HV rectifier from a 12 inch
CRT monitor was used for isolation with a 100K resistor for current limiting.
The current could be varied from 6.0 mA (provided by the brick) and 7.0 mA
or more by adjusting the auxiliary supply.

It would be a simple matter to construct an auxiliary power supply with
a low voltage DC input using a 555 and MOSFET to drive a small flyback.
A duty cycle/frequency adjust pot would permit output current to be easily
varied. This could turn an otherwise not terribly useful fixed current
brick into a very flexible lab HeNe power supply.

While less common than the case above, there may come a time when it would be
nice to operate a 7.0 mA power supply with a laser tube requiring only 6.0 or
6.5 mA.

In principle, this could be done by adding a current bypass in parallel with
the HeNe laser tube. If it weren't for the issue of the very high starting
voltage, a simple resistor (rated for high voltage) would be adequate. For
a known voltage across the laser head (tube + ballast resistance), Ohm's Law
could be used to determine the resistance value to use. For a small
reduction in current (e.g., 0.5 or 1 mA), no regulation would be needed.
However, the problem is how to deal with the starting voltage. Any resistance
connected permanently across the laser head would probably prevent the
starting voltage from being high enough due to the very limited current
availability of most starting circuits. One way around this would be to
add a high voltage relay that only closed the current bypass after the
tube has started. However, it probably would need to be a mechanical relay
and one rated for 10 kV or greater. A manual HV switch could also be used.

HeNe laser power supplies can be used for non-laser experiments as long as
they are not called upon to operate outside their normal compliance range.
Depending on model, you can get anywhere from 1 kV to 6 kV (or more) at a few
mA. One such application is as a capacitor charger for small pulsed solid
state lasers. See the chapter: Complete SS
Laser Power Supply Schematics for a specific example.

Put a load on the output so that it runs in the normal operating (not starting)
range of voltages. Near the upper end is fine (e.g., higher load resistance)
as long as the voltage remains within specs but you don't want to have the
supply continually attempting to start a non-existent HeNe tube - that is hard
on any supply.

Note that most HeNe laser power supplies including 'bricks' are NOT
totally isolated between input and output - the negative of the high voltage
is probably either directly or via some high resistance connected to the
earth ground (AC line types) or the negative of the input (DC input types).
Check the continuity between the black HV lead and the negative of the input.
Many are tied together internally. Even if it isn't 0 ohms, there may be
current sensing feedback that is via high value resistors.

Ballast Resistors, Function, Selecting

Every HeNe laser (as well as any other gas laser, fluorescent lamp, xenon arc
lamp, or other similar device) needs components between the actual voltage
source and the tube to stabilize the discharge and limit/regulate the current.
Without a ballast, the resulting discharge would be flashing, flickering, or
unstable in some other way AND the tube and power supply would likely destroy
each other due to excessive uncontrolled current flow. For high power devices,
the ballast may take the form of an inductor or transformer, or an electronic
circuit that performs an equivalent set of functions. For the HeNe laser, the
ballast is almost always a simple resistance. The purpose of the ballast
resistor (Rb) in a HeNe laser is twofold:

Assure stability: Rb used to adjust the total resistance in the HeNe
tube circuit so that it is comfortably positive. This is needed for
stability to balance the negative resistance characteristics of the HeNe
tube itself.

The second function depends to some extent on the type of power supply:

For power supplies that are not current regulated, Rb serves to limit the
HeNe tube current to the optimal and safe value for proper operation with
the desired input voltage to the power supply (e.g., 115 VAC). Although
the size of Rb doesn't directly affect laser output power, it may control the
current and output power for most HeNe lasers peaks at a specified current -
above or below this optimum value and it goes down. Thus, unlike the case
of a resistor in series with an incandescent lamp, reducing the value of
the Rb may in fact reduce laser output.

For power supplies that are current regulated, the value of Rb can be
chosen to place the operating point near the middle of the compliance range
of the regulator. This will maximize immunity from input variations and
ripple, and may reduce stress on regulator components as well.

Selecting a ballast resistor that works with a given tube is usually a trivial
exercise. So, don't let the length of the following discussions intimidate
you! It is quite possible that for the entire future of the universe (until
the big crunch and even beyond), you will have no need to use any other value
than the usual 75K ohms. However, there are situations where this will not
work well or at all when mating a power supply to a HeNe tube or laser head
other than the model or size for which it was originally designed.

If you are connecting a power supply to a laser head (not just a bare tube),
there is almost certainly a ballast resistor inside the head itself. So, an
additional full ballast resistor will not be needed unless you find that the
original value is insufficient. Also, the power supply may already have some
amount of ballast resistance (partial or up to 75K) and the combination may be
too large! When the designation Rb is used, it means the *total* ballast
resistance but this may actually be made of up 2 and sometimes even 3 parts.
See the section: Locations of the Ballast
Resistance.

Note: Although a single resistance value may be specified below for Rb, it is
better to construct the actual ballast resistor from several lower value
resistors in series to achieve the required voltage and power ratings.
See the section: Power Supply Construction
Considerations for more info.

Some of the procedures below may require measurement of voltage or current
in the HeNe tube circuit. Before proceeding, for your own safety and the
continued good health of your test equipment, see the section:
Making Measurements on HeNe Laser Power
Supplies.

As noted, simple neon signs and virtually all low current gas discharge
devices also have a negative resistance and require a ballast to operate
in a stable manner. For neon signs where there is no ballast resistance,
this function is implemented inside the magnetic or electronic neon sign
transformer - which is often called a ballast. These will have a high
effective impedance by their design.

Implementing the ballast inside the transformer is also a lot easier
for neon signs since they run on AC current and inductance can be easily
used in place of resistance. For HeNe lasers, the high impedance would
have to be implemented with active electronics. Current regulated HeNe
laser power supplies do have a high impedance but it's never depended on
for stability. With unspecified wiring and tube capacitance, it's just
a lot easier and cheaper to depend on a simple ballast resistor and accept the
modest waste of electrical power. The benefit is that any HeNe laser
tube can be mated with any power supply with compatible voltage and
current specifications without worrying about details. For fluorescent,
high intensity discharge, and arc lamps that require much higher current,
a resistive ballast would waste a lot of power. But the 3 to 8 mA of a
typical HeNe laser means 10 to 25 percent of the total power is wasted
in the ballast, not really very significant when the overall efficiency
(wall plug power to optical output power) of a HeNe laser is already so
abysmal - 0.01 to 0.1 percent! If efficiency is important, use a diode
laser. :)

If you examine a commercial HeNe laser, unless the HeNe tube is physically
within a couple of inches of its power supply, there is a good chance that the
actual components that comprise the ballast resistance are split into 2 and
possibly even 3 separate sets of physical resistors:

(Optional current sense resistor and other additional components not shown.)

Rba - Generally, most (or all) of the ballast resistance is located as
close as possible to the HeNe tube anode in the laser head. Its value is
typically 75K, with a power dissipation of about 1 W/10K. In some cases, a
smaller value may be acceptable if the rest is made up by Rbp or Rbc (see
below) and the distance between the laser head and power supply isn't too
great. Some lasers may use a higher value as well. If the tube and power
supply are close together, Rba is likely the only ballast resistor present.

Rbp - One part may be physically located between HV+ (see the section:
Basic HeNe Laser Power Supply Organization)
and the high voltage output (Alden) connector of the power supply. Important
functions of Rbp are to provide protection for the power supply in the event
of an external short circuit, make up a portion of the positive resistance
needed to stabilize the discharge, and move some of the heat dissipation to
the power supply which presumably can better deal with it. The typical value
of Rbp is 20K to 50K with a power dissipation of about 1 W/10K. However, as
far as HeNe tube operation is concerned, Rbp may be omitted entirely as long
as Rba is of adequate value (usually at least 75K).

Rbc - Some commercial laser heads may have a small ballast resistor
(typical value of 10 to 15K) in the cathode return circuit as well.
This can often be eliminated without any ill effects but some, mostly
larger, HeNe laser tubes will not operate in a stable manner without
some cathode ballast resistance, particularly if installed in
a metal enclosure like a cylindrical laser head. The symptoms are
that the laser would sputter or randomly or repatedly restart, possibly
after being on for awhile.

If it weren't for stray capacitance, only the total ballast resistance, Rb,
would matter with respect to discharge stability and current regulation. If
the power supply and tube are within a couple of inches of one-another, this
IS all that really matters. However, in the real world, where remote laser
heads are often desirable, it isn't always that simple! But, dealing with Rb
as though it were a single resistor as a starting point won't hurt. Thus,
unless otherwise noted, the discussions below refer to the total of all
resistance in series with the HeNe tube even if Rb is not physically at a
single location.

Using the actual HeNe tube (rather than a dummy load) is generally the
easiest, fastest, and most accurate approach for selecting the ballast
resistor. Unless the power supply and HeNe tube are grossly mismatched,
the chance of damage to either is small. Here are several possibilities:

If your power supply is variable, or current regulated and reasonably well
matched to the size of the HeNe tube, a 75K resistor will almost always work
just fine. This is the value that is likely to be inside a laser head as
well. That's all there is to it!

While the theory is complex, the result is simple: It turns out that the
value of the *negative* resistance of a typical HeNe tube around its optimal
operating point is usually approximately -50K. The reason is that larger
HeNe tubes tend to have longer but wider bores and the effects of these tend
to cancel out. For a stable discharge, the value of the total effective
resistance must be positive - else you get a relaxation oscillator. Using a
75K ballast resistor generally provides adequate margin without dissipating
more power than needed.

For a regulated power supply, the optimal ballast resistor will operate the
tube near the middle of the voltage compliance range of the regulator. This
will result in the best immunity from input variations and ripple. However,
there is a lot to be said for the recommendation: "If it works, use it."!
So, start with 75K and if it works, leave it alone. Should you really want
to spend time on getting to know your power supply, see the section:
Determining the Compliance Range of a Power
Supply.

However, with some HeNe tube-power supply combinations, 75K may still be
too low to maintain a stable discharge at the recommended tube current - or
the current will be too high (particularly where a large power supply is
being used to drive a low power HeNe tube). In this case, it will be
necessary to increase the ballast resistance to 100K, 150K, or even more,
and accept the additional heat dissipation and wasted power.

For a power supply without current regulation, if you know or can estimate
the operating voltage of your HeNe tube (Vo) either from its specifications
or by comparing it to those listed in the charts in the section:
Typical HeNe Tube Specifications AND know
the approximate output voltage (Vout) of your power supply at the operating
current (Io), use the equation:

Rb = (Vout - Vo) / Io;

to estimate the ballast resistor value. If you do not know Vo and Vout,
start with a value of 75K.

If the HeNe tube current is to large, Rb will need to be increased.
Where the power supply and HeNe tube are grossly mismatched, it may need
to be quite large (e.g., several hundred K ohms).

If the HeNe tube current is too small, Rb will need to be decreased.
However, going much below 75K will likely result in instability. The
power supply is probably inadequate for you HeNe tube in this case.
However, it won't hurt to try - some tubes will be perfectly happy with
a 50K ballast but the setup may be more susceptible to input voltage
fluctuations if there is little or no regulation.

Another similar approach is to use a variable input source (Variac or
adjustable DC power supply as appropriate) with an initial Rb of 75K.
Increase the input until the tube starts and back off if necessary to the
minimum at which the discharge remains stable. Then, increase Rb as above
until you can obtain the desired tube current at the nominal input voltage.

If there is no input voltage at which the tube starts and the discharge is
stable, 75K may already be too large and it is likely that the power supply
is inadequate for your HeNe tube.

Optical power output from most healthy red (632.8 nm) HeNe lasers peaks
at the optimum value of tube current. Therefore, selecting the ballast
resistor can even be done by (reasonable) trial and error by simply
maximizing output beam brightness without worrying about current
measurements! Note that it takes a few seconds when changing current
for the output power to stabilize so be patient if attempting to do it
this way.

Tube current below optimum will result in slightly reduced optical output.
However, it is not usually possible to decrease current below about 50
percent of optimum (often not even this low) and maintain a stable
discharge. Even at these cutoff currents, output power is usually only
down slightly.

Tube current somewhat above optimum will result in slightly reduced
output power and increased optical noise. This won't be noticeable by
eye but could be significant if the laser is being used as part of a
communications or measuring system. There will also be excessive heat
dissipation and a possible reduction in tube and/or power supply life.

Tube current above 2 to 3 times optimum will result in NO output beam at
all (but it will recover instantly as long as the tube isn't left to cook
too long with this current). However, if the HeNe tube is good, there
would probably be a flash of laser light from its output at power-on
and/or power-off as the tube current passes through the normal operating
range.

The exceptions to the optimum power rule are for (1) "other color" HeNe lasers
filled at reduced pressure and/or with higher He:Ne fill ratio and (2) for
some wide bore multiple (transverse) mode tubes where the output power may
continue to increase up to a much higher current, possibly beyond what is
safe to use with the tube. In addition, (3) as a HeNe laser tube is used, its
characteristics will change and near end-of-life tubes or soft-seal tubes
which have leaked may peak at a higher and possibly excessive current
as with (1) above.

Don't get carried away - running the tube with slightly excessive current
won't damage or destroy it immediately (unlike a laser diode). However,
the ballast resistor may not survive overcurrent for long since its power
dissipation goes up as the square of the current. (The HeNe tube power
dissipation only goes up slighly less than proportional to current due to
its negative resistance characteristic.) Also, using a regulated power supply
near or beyond one end or the other of its rated voltage compliance range
could result in overheating or overstress of components and it may be damaged
or fail completely. Thus, a regulated power supply designed for a .5 mW tube
should generally not be used with a 5 mW tube (or vice-versa) for any length
of time even if it appears to operate properly!

Here are a couple of approaches to selecting the ballast resistor for a small
HeNe tube and simple unregulated power supply like the one described in the
section: Edmund Scientific HeNe Laser Power
Supply.

(From: Steve Nosko (q10706@email.mot.com).)

The ballast resistor, the actual voltage at the doubler output and the tube
running voltage determine the tube current. Look at the tube ratings for the
running voltage and tube current - 1,150 V at 4 mA for the tube I was using.

One can approach the design from two directions:

With a supply of 1,750 V and a tube voltage of 1,150 V. this leaves 600 V.
across the ballast resistor. Once you get the transformer picked out, make
your best estimate of the output voltage you'll have. It should be above
1,400 V for a 0.5 mW tube. Any less and you could get into trouble since the
ballast resistor should be kept above 50K. Some sources recommend 70K as an
optimal value for a .5 mW tube.

Working the other way. First calculate the ballast resistor voltage drop
as, V = tube current * 70K. Add this voltage to the tube voltage to get the
required supply voltage. This would be a minimum supply voltage for that
tube. If yours gives a few hundred volts more, you can make the ballast
resistor larger to obtain the required tube current. It's just that the
bigger the ballast resistor, the more power you throw away. If you think
you may go to a more powerful tube in the future, then get a higher voltage
transformer output for the bigger tube and just use the larger ballast
resistor for now.

This approach will also permit the limits or compliance range of the power
supply to be determined without risking the HeNe tube. This may be necessary
to select the ballast resistor to match a HeNe tube up with a power supply
that is not marked or determine its limits - what sizes of HeNe tubes it will
drive. Even when marked, the ratings on a typical HeNe laser power supply do
not tell you how it will behave under varying load conditions. However, first
see the section: Selecting the Ballast
Resistor as the approaches described there are usually adequate for the
vast majority of situations.

Where the voltage drop across your HeNe tube is known (from its specifications
or having been measured), it can in principle be replaced by a fixed resistor
equal to Vop/Iop for the purpose of basic power supply testing. (However,
dynamic behavior will be affected by the HeNe tube's negative resistance
characteristic. This is probably not a problem as long as the resulting
ballast resistance ends up greater than that magic 75K ohms.) For example,
a HeNe tube rated at 4.5 mA at 1,200 V can be replaced by a 267K resistor
at its operating point. Then, enough ballast resistance is added beyond 75K
so that the proper current (4.5 mA in this case) is obtained.

Using a dummy load will also permit a ballast resistor value to be selected
such that the regulator operates near the middle of its compliance range.
This will provide the maximum tolerance of low and high input (AC or DC)
voltage (input compliance) and slight differences among HeNe tubes and/or
conditions as the tube heats up and/or component values drift with age and use.

CAUTION: Determining the limits of some power supplies is like testing a new
jet fighter; pushing the envelope entails some risk as the power supply can be
damaged or destroyed if run outside its design specifications. This is
especially true with power supplies that are current regulated. Damage could
result if the design is not adequately protected. Many commercial designs
are quite robust but don't push your luck! If your power supply is current
regulated, see the section: Determining the
Compliance Range of a Power supply.

Use a voltage divider constructed from a 500K potentiometer (actually four
100K (5 W) resistors and five 20K, (1 W) resistors to get high enough
wattage - not an actual pot since such things probably do not exist), 75K
(5 W) resistor and 1K (sense) resistor.

This load will short out (and thus disable) voltage multiplier type starting
circuits or fool any auto start circuit into thinking the tube is running.

These resistor values should work for most tubes rated between 1100 V and
2,400, and currents between 4 mA and 8 mA with typical power supplies but
yours may be the exception. Therefore, the resistor values may need to be
adjusted if you cannot obtain meaningful results.

Measure the voltage across Rs to determine current. The sensitivity will be
1 V/mA. Alternatively, simply put a 10 or 20 mA current meter across or in
place of Rs. (Then, Rs is effectively 0 for the calculations, below.)

Start with R1 at its maximum.

Apply power and measure the current (I) through Rs as described above.
Your goal is for this to be equal to the nominal HeNe tube current (Io).

Kill power and confirm that any capacitors have discharged before touching
anything!

If the measured current is still too low, reduce R1 and try again (goto
step 2).

Calculate or measure the resistance of the entire R1 assembly. The value
of the ballast resistor (Rb) should then be:

Determining Characteristics, Fixed Current Considerations

Most commercial HeNe laser power supplies incorporate some type of regulator to
maintain the current through the tube constant. It may be fixed at a set
value like 5 mA or 6.5 mA or may be variable via a front panel or internal
pot, by changing a resistor somewhere, or even by a control voltage (providing
modulation, for example).

By varying the load using the test circuit (above), and taking measurements of
both voltage and current, the limits of such a regulated power supply can be
explored. Over the useful compliance range of a power supply, output voltage
will automatically vary to maintain the output current nearly constant
independent of input and load variations and ripple from the power supply
itself.

However, since the HeNe tube introduces some negative resistance into the
equation, actual performance may not be quite the same as with the dummy load.
Thus, although the power supply may be able to maintain a specified current
over a certain range of dummy load resistance values, some portions of this
range (usually at either end) may not result in stable behavior with actual
HeNe tubes.

CAUTION: There is always some risk of damage if the specifications are unknown
and the designers didn't provide adequate protection. Keep this in mind if
you decide to test your power supply in this manner.

Power supplies with linear regulators are prone to failure when they are
called on to drive a load resistance that is too low because the voltage
drop across their regulator may become too great and parts may than short or
open. Thus, attempting to drive too *small* a HeNe tube may cause problems
with these types of supplies - and there may be no warning before the
regulator calls it a day. The result may then be *no* regulation and
greatly excessive current which is limited only by the power supply's
effective series resistance.

If the load resistance is too high, these types of supplies simply do not
regulate properly and the output current will be reduced. Where the voltage
drop across a (large) HeNe tube is too great, it will not operate properly.
However, damage to the power supply itself is unlikely unless left in this
condition for a long time.

For power supplies with linear regulators, the safe limits are between 0 and
Vsafe volts across the regulator components where Vsafe is less than the
specified breakdown voltage of the pass transistor(s). With better designs,
Vsafe will be restricted in other ways for protection but the output current
may increase once Vsafe is exceeded. The actual output of the power supply
will have a compliance range between (Vmax-0) volts and (Vmax-Vsafe) volts
where Vmax is the output voltage under the specified load but with the
regulator short circuited.

For inverters which adjust pulse width or frequency to regulate output
current, the usable limits will be determined by the range over which the
controller can properly drive the switching transistor. These may fail when
they are called on to drive a load resistance that is too large because
their switching transistor and other components may be overstressed or
overheat.

Without knowing the design in detail, there is really no risk-free way of
determining this on the high end though better designs will limit the
drive to a safe value before parts blow up. As the load resistance is
increased, a point will be reached where regulation is no longer possible.

Problems are most likely when driving HeNe tubes that are too large. The
supply will be operating at or beyond its maximum specified output voltage
which may result in a dead supply if left that way for too long. There may
be no warning, or the HeNe tube may blink or flash, behave erratically, or
not start at all - these conditions are stressful for both the power supply
and tube.

What happens with too small a load resistance (or too small a HeNe tube) is
more difficult to predict but could there could also be problems. At the
low end, the current will start to rise if the drive pulse width cannot be
reduced sufficiently. This is analogous in some ways to the minimum load
requirements of a typical switchmode power supply (which is essentially what
these are).

CAUTION: For regulated power supplies (inverters in particular), only leave
power applied long enough to take your readings if the current is not being
maintained at the specified value as this may be stressful to some designs.
Where the specifications include a voltage rating (e.g., 2,500 V), this is
generally the maximum that should be output continuously. In this case, the
upper limit is already known - going beyond this is asking for trouble.

Start with R1 at a modest value and work first one way and then the other
so that the limits in both directions can be determined. A suggested
initial value for R1 is around 250K. This is likely to be within the
compliance limits of typical power supplies for small to medium size (i.e,
.5 to 5 mW) HeNe tubes).

Apply power and note both the current (I) through Rs and the total voltage
(Vout) from the power supply.

CAUTION: For power supplies with linear regulators, it is best to use a
variable input source and bring up the supply slowly while also monitoring
the voltage across the pass transistor(s) to assure it doesn't begin to
approach their breakdown voltage. If this happens, R1 is too small (beyond
the lower limit of compliance).

Current can be measured as described above. Vout can be measured if you
have a voltmeter with a high enough range or calculated as:

Vout = (R1 + 75,000 + Rs) * I;

There will be three possibilities (Io is the value of current that the
power supply is supposed to be maintaining):

If I = Io, the power supply is within regulation.

If I < Io, R1 is (or has become) too large.

If I > Io, R1 is (or has become) too small.

Kill power and confirm that any capacitors have discharged before touching
anything. Modify R1 as appropriate and goto step 2. You are finished if
the entire range over which current is nearly constant has been explored.

The compliance limits are at the points where the current goes out of
regulation at each end of the range of values of R1. As noted above,
whether these are actually usable limits depends on design and your
particular HeNe tube.

The HeNe tube voltages (range of Vo) that will be supported (assuming a 75K
Rb) will be within the limits:

(R1(u) * Io) > Vo > (R1(l) * Io);

where R1(u) is the value of R1 at the upper end and R1(l) is the value of
R1 at the lower end of the range where the current (I) is constant.

For a given HeNe tube with specified Vo and Io, the Rb which has the power
supply in the center of its compliance range is given by:

Rb = (((R1(u) + R1(l).)/ 2) + 75,000 + Rs - (Vo / Io);

As noted above, for regulated supplies, the current will be maintained at
nearly a constant value over some range of output voltage. Therefore, if
possible, select the ballast resistor (Rb) for your particular HeNe tube such
that the supply is operating near the center of its voltage compliance range.
Of course, where a large tube is used on a smaller supply or vice-versa, the
usable compliance range will be reduced. In fact, there isn't even any sort
of guarantee that the 'optimal' Rb value calculated above will even work with
your HeNe tube! Life it not simple. :-)

An alternative is to perform the compliance tests with a HeNe tube installed.
The problem with this is that starting may make measurements more difficult
and result only applies directly to the HeNe tube that is used. However, this
IS a more accurate procedure. The series resistance is varied in the same
manner as described above except that it will much smaller since much of the
voltage drop is taken up by the HeNe tube. The modified setup is shown below:

The procedure is otherwise similar but take care when making measurements
since the starting circuit is no longer disabled and use an initial R1 of 25K
instead of 250K. As noted above, if you had previously determined the limits
of your power supply using the dummy load, with the HeNe tube installed, the
discharge may become unstable before these limits are reached. Details are
left as an exercise for the student. :-)

Many inverter type power supplies are completely potted (i.e., 'bricks') and
will have a fixed current specification (e.g., 5 mA, 6.5 mA) and a maximum
voltage specification (e.g., 1,750 V, 2,500 V). They will generally work with
a 75K ballast resistor and attempt to maintain their rated current through a
variety of HeNe tubes (various sizes and power ratings). Unless there is an
adjustment accessible through an access hole, there is no easy way change this
current set-point.

CAUTION: These power supplies may attempt to maintain the set HeNe tube
current even when the operating voltage is beyond their specifications as
might be the case with a high power HeNe tube on a small power supply or a
ballast resistor that is too large. The power supply may overheat or be
overstressed under these conditions and fail without warning. See the
section: Typical HeNe Tube Specifications to determine if your HeNe tube
is likely to be within the capabilities of your power supply brick.

Where the current rating of your HeNe tube does not match the power supply
(and it cannot be adjusted), your options are limited. There is no easy way
to increase or decrease current with external circuitry. However, if it is
within +/- 10 percent of the optimum current, don't worry about it. If the
current is within +/- 20 percent of optimum, it still isn't the end of the
world (though a different power supply would be the best solution):

Where the current is too low, power output may be reduced slightly. If
the discharge is stable (not flickering or flashing), live with it.

Where the current is too high, power output may be reduced slightly as
well. There will be greater power dissipation at the HeNe tube anode and
this may reduce tube life. I do not know if this is that significant.

Power Supply Measurements, Testing, Repair

Voltage and/or current measurements on a HeNe laser power supply may be needed
to characterize its performance bounds, to troubleshoot or identify a defective
unit, or to monitor conditions during operation or with different HeNe tubes,
particularly where there are user adjustments (i.e., a Variac used as an input
voltage source).

Generally, under normal conditions, the current is what is important.
Fortunately, current measurements can usually be carried out easily and
safely. These will also readily identify a reverse polarity situation.

However, when troubleshooting a misbehaving power supply or determining
the bounds of its characteristics, voltage measurements may be needed.

Note: I DO NOT recommend the use of typical DMMs (Digital MultiMeters) for
measurements in high voltage power supplies of this type. Many of these are
more susceptible to damage from voltage or current spikes than analog VOMs
(Volt Ohm Meters) or simple dedicated moving coil (D'Arsenval) panel meters.
However, even with these, there can be internal arcing which may result in
damage that doesn't show up until long after you have forgotten all about HeNe
laser power supplies! :(

It is difficult to measure the output voltage of a HeNe laser power supply
with a multimeter even if it is supposedly within your meter's range.
Connecting the meter across the tube while it is on will likely extinguish
the arc due to the capacitance of the probe inducing a momentary dip in the
voltage. Even if the tube remains lit or restarts, there may actually be
oscillation resulting in an erroneous reading. Where possible, measurements
should be made upstream of the ballast resistor(s) and corrected for their
voltage drop.

Leaving a multimeter connected during starting may prevent the tube from
firing due to its additional capacitance and reduced resistance. And, the
meter may be damaged due to arcing from any high voltage starting pulses.

Here are some recommendations:

Voltage: Home-built power supplies with separate starting circuits provide
options usually unavailable with modern commercial units. The voltage
upstream of the starter is relatively stable and well behaved (at least it
should be) and should not damage the multimeter as long as it has adequate
range. However, a slight offset may need to be subtracted from your
readings if there are high voltage diodes in series with the output (as with
many starting circuits). However, high compliance designs do not offer this
option.

Commercial supplies may not provide access to any convenient voltage test
point. It may be possible to measure between the power supply side of the
ballast resistor and the HV return ONCE THE TUBE IS RUNNING. However, this
is risky - if the tube goes out, the starting voltage will appear at this
point and may fry your meter or find a convenient path to ground through YOU.

A VOM or DMM with a suitable high voltage probe can be left connected to a
wide compliance type power supply and possibly on one using a voltage
multiplier (though it may load it excessively) but should probably not be
used with a pulse (trigger) type starting circuit.

Another approach is to leave a high value resistor - say 10M ohms - attached
to the power supply but disconnected from your meter while starting. Once
the tube is lit, carefully measure between the resistor and the tube
cathode. The high value resistor should minimize any transient when probing
and prevent the tube from going out. Then, correct for the additional
resistance in series with your meter. For example, with a 20K ohm/V VOM on
the 2,000 V scale, the meter resistance is 20M ohm so the reading would need
to be multiplied by 1.5. If the measurement is made in front of the ballast
resistor(s), use Ohm's law to determine the actual tube voltage (if you care)
based on the tube current (see below) and ballast resistance (assume 75K if
inaccessible).

Measuring the starting voltage can be difficult depending on the type of
circuit used in your power supply. See the section:
Testing a HeNe Laser Power Supply for more
information.

Where you are interested in AC voltage (i.e., ripple or noise), couple the
test point through a HV coupling capacitor to block DC. Its voltage rating
must be adequate to hold off the maximum possible output of the power supply
and its uF value must be large enough to minimally affect the measurement
accuracy. At 60 Hz, for example, a .01 uF capacitor is large enough to
to produce less than a 5 percent error on a 10 M ohm input impedance DMM.
CAUTION: Provide some sort of excess voltage or surge protection (e.g., an
NE2 neon bulb) across the inputs to the multimeter or scope so that multi-kV
transients don't find their way into your test equipment!

I use a series combination of a 20K ohm HV resistor, 1 nF, 15kV capacitor,
and an NE2 for protection to limit the voltage on the scope input to about
90 V. The impedance of 1 nF at 60 Hz is about 3M, so the result is a slight
attenuation (around 25 percent).

Current: In all cases, a meter can be placed in the RETURN circuit of the
HeNe tube. At this location - which should be safely near ground potential
if at all possible - the capacitance of the meter and probes will not affect
starting or operation in any way. Reverse polarity due to incorrect wiring
will be instantly obvious. The measuring device can be a VOM or DMM (though
as noted, I do not recommend DMMs), or a simple panel meter (which doesn't
tie up your expensive multimeter):

Obtain a 10 mA panel meter (I use a surplus Weston - it probably dates from
the 1950s). Mount it in a well insulated or grounded case, and add a set of
well insulated color coded (red and black for + and - respectively) high
voltage leads with banana plugs on the ends. Then, any power supply should
include a 1K resistor in series with the cathode return connected to jacks
on the case. This 'current sense' resistor (Rs) will have no affect on
power supply performance but will prevent any significant voltage from
appearing at the test jacks if the meter is not present. The 10 mA meter
will effectively short out Rs so essentially all the currennt flows through
it. A voltmeter can be used instead of a current meter across Rs. The
sensitivity will then be 1 V/mA. Either type of meter can be left in place
permanently if desired. If you expect to be testing many HeNe lasers,
get male and female Alden connectors and wire up the meter permanently
between the negative/cathode (black) wires. Solder the red wires together
insulate them most extremely well. :) Aldens may be salvaged from dead
laser heads and power supplies, or may be purchased from various surplus
places.

Some commercial laser power supplies already have a built-in sense resistor
in an easily accessible relatively safe location for current monitoring or
you can easily add one.

Where you are building your own power supply, make sure it has its negative
output earth grounded (3 prong line cord or separate wire screwed to a
suitable ground) if at all possible. This will assure that the cathode end
of the HeNe tube, metal parts of the laser head, and current measurement
test points are all at or very near ground potential - and thus less of a
hazard should you touch any of them (though I am not recommending this!).

For commercial units, test to see if this is already the case or is possible
(it almost always is but there are no doubt exceptions). Such precautions
will greatly reduce the chances of shocking experiences since the only part
of the laser head at a high potential will be the ballast resistor and tube
anode.

WARNING: Make sure whatever you have is well insulated and/or grounded where
appropriate. Those high voltages can bite! Use proper high voltage cable
(rated 20 kV or more - non-resistive type automotive ignition cable and TV
or monitor CRT high voltage cable works well) and insulated coverings on the
terminals (several layers of plastic electrical tape or Plexiglas barriers).
You may be working in the dark (light-wise, at least) or with somewhat subdued
lighting, so it makes sense to prevent accidental contact as much as possible.
This is especially important with power supplies that may be overkill (no pun
intended...really!) as is often the case with home-built equipment.

CAUTION: Assure that all connections are secure and that the high voltage
connections are well insulated. Intermittent contact, arcs, shorts, and other
faults may be fatal to power supplies regardless of what their Marketing
blurb says about being *fully* protected!

I built this hand-dandy gadget both as a means of easily adjusting the ballast
resistance as well as providing a convenient readout for HeNe tube current
and voltage. It is shown in HeNe Laser Ballast Resistor
Selector and Meter Box. The entire complex circuit is mounted between a
pair of blank Fiberglas-Epoxy PCB panels separated by 2 inch standoffs. It
will plug into any HeNe laser power supply with an Alden connector and has
special custom clip leads (read: alligator clips with formed paper clips
soldered to their ends) to easily attach to the mirror mounts or HV terminals
of any HeNe tube.

If you're wondering about the peculiar resistor value of 27.1K, the answer
is the same one as to the question: "Why do people climb mountains?". :)

The HV leads plug in via banana jacks and can be either a short clip lead
for the tube anode and longer one for the tube cathode, or a female Alden
connector (as shown in the photo). The Alden positive lead can also be
plugged in to any of the non-zero ballast resistance jacks to add ballast
resistance if desired.

Note that (except for the 27K setting), the Tube Anode lead always goes
directly to the end of exactly one resistor to minimize capacitance at the
tube and to protect the power supply in the event of a, well, unfortunate
accident (like a short circuit). (For the 27K setting, the assumption is that
there is an additional ballast resistance near the tube anode so capacitance
doesn't matter as much.) This lead is also relatively short - about 4 inches
for this same reason. Each tap is a banana jack and the tap selector is just
a VERY WELL INSULATED jumper with a banana plug on its end (though I still
don't recommend trying to change settings while powered!).

The following is connected directly across the output for measuring tube
voltage:

Rm1 and Rm2 are high voltage resistors. S1 is actually just another jumper
and banana jack (the short red one in the photo, shown in the
closed position) since a common switch would need to be capable of withstanding
over 10 kV during starting. The purpose of S1 is to unload the HeNe laser
power supply during starting. However, if your power supply has enough
current capacity while starting (usually high compliance inverter designs
as found in most bricks, not
parasitic multipliers), it may be possible to keep the meter in the circuit
all the time. Just make sure your components are adequately rated for the
maximum starting voltage which may be present if a HeNe laser tube is not
connected or simply refuses to start. The DMM will read 1/10th of the actual
tube voltage. Even if the tube cuts out and tries to restart, the DMM should
survive but I can offer no guarantees for your specific implementation!
However, putting a series string of NE2 neon lamps across the meter would
be good insurance - figure 90 V for each NE2 so add enough so that they
will start conducting when the meter exceeds its full scale voltage.

I normally use this rig with an Aerotech PS2B HeNe laser power supply which
has had its regulator bypassed. (See the section:
Aerotech Model PS2B HeNe Laser Power Supply
(AT-PS2B).) No matter what protection there is,
semiconductors tend to blow themselves up to protect the fuse. :) And for
testing there is really no need for great regulation. It has been installed
in the case from an Aerotech LS4P laser (which is 3 to 4 inches longer than
the PS2 case) so a small Variac fits snugly in one end to adjust tube current.
It may be set low for 0.5 mW barcode scanner HeNe tubes or cranked up to an AC
input of 140 VAC for dealing with hard-to-start tubes, or to drive any
HeNe laser tube or head of up to at least 12 mW on the bench while
monitoring current and voltage. (The modified
PS2B is quite happy running at 140 VAC input all day but this may not be true
of some other power supplies.) The combination of the PS2B, tapped ballast
resistor, and metering system is my workhorse for testing HeNe lasers.

If you're characterizing a few thousand HeNe lasers of various sizes,
there is at least one tester on the market designed to make your life
easier (and pocketbook lighter!). The
Voltex, Inc. U-40 (about $600) can
power virtually all HeNe lasers and includes current adjust, and voltage
and current monitoring.

Failure of a HeNe laser to lase could be due to a bad tube, bad power supply,
bad connections, or you forgot to plug it into the wall socket!

If there is a continuous glow from the inside of the HeNe tube, the power
supply is probably working properly though the current could be incorrect.
However, this would result in reduced output power or excessive heating - but
not a totally dead laser unless the current was more than 2 or 3 times too
high. (In this extreme case, if the tube is good, there would likely be at
least a flash of laser light from its output at power-on and/or power-off as
the tube current passes through the normal operating range).

Where there is no sign of a discharge - even momentarily - the problem could
be with the tube, wiring, or power supply.

If the laser is flashing or sputtering, or the current is way too high
(based on measurements), the tube is either broken or its voltage requirements
are outside the compliance range of the power supply at its present current
setting. If the current is too high, the tube voltage is probably too
low. If it flashes or sputters, the tube voltage may be too high.
Either condition is very hard on power supplies and failure may not be
far behind. So do not push your luck and find out what's wrong.

In either case, it would probably be a good idea to see the section:
How Can I Tell if My Tube is Good
before getting into heavy troubleshooting of the power supply.

To determine if the power supply is working requires testing it for both the
starting and operating voltage (and proper current if it includes an internal
regulator). Of course, the easiest approach is to substitute a known working
HeNe tube, but this is not always a viable option. If all that is desired is
to determine if the power supply is not dead and no test equipment is
available, see the procedure at the end of this section.

Testing to determine if the power supply is producing the proper operating
voltage can be done by substituting a resistive load for the HeNe tube (and
ballast resistor). Where the specifications are known, use the operating
voltage and current to select a suitable power resistor from: R = Vo / Io.
Where no such data is available, estimate the specifications by locating a
HeNe tube in the section: Typical HeNe Tube
Specifications similar to the one you will be using. Make sure the
resistor can handle at least twice the expected current (just in case).

This load will short out (and thus disable) a voltage multiplier or fool any
other type of starting circuit into thinking the tube is running.

Add a 1K ohm resistor in series with this load to use for measuring the
current (and from this, the total voltage from the power supply).

As an example, for a typical 1 mW HeNe tube requiring 3.5 mA at 1.4 kV, the
load resistor (Rl) should be 400K ohms. The power dissipation at this
operating point would be about 5 W. Use several lower wattage resistors
in series (to handle the expected maximum voltage) to make up the required
resistance for R1 with a total power dissipation of around 10 W.

Measure the voltage across Rs to determine current. The sensitivity will be
1 V/mA. Alternatively, simply put a 10 or 20 mA current meter across or in
place of Rs. Power supply output voltage is then Vo = (Rl / Rs) * V(R2).

If your measurements indicate that voltage and current are 0 or much much
less than expected, the power supply is bad, your input voltage is
incorrect or miswired (e.g., you are using the 230 VAC rather than the 115
VAC wiring), or there is an enable input (logic signal) which is not
connected or set to the wrong state.

If your readings are somewhat low, your documentation may be incorrect
or the load resistor may be too high for the power supply and it is not
capable of providing both the expected voltage and current.

If your readings are too high, your documentation may be incorrect, or the
load resistor may be too low for the power supply and it is incapable
of reducing both the voltage and current far enough.

If the power supply has a current adjustment, see if this behaves as expected.
A control that does nothing could indicate that the load resistor is sized
incorrectly and the compliance range of the power supply is being exceeded
(low or high).

In all cases, a defective regulator or control circuit could result in these
faults as well.

A stack of NE2 (or similar) neon indicator lamps (without built in current
limiting resistors) AND a 75K ohm (5 W) ballast resistor (Rb) in series can
provide a rough indication of proper operation if you don't have test
equipment. Each NE2 has an operating voltage of about 60 V at 1 mA but this
will increase by about 25 percent at 6 mA (and NE2s probably don't have a
very long life at 6 mA). The starting voltage is about 90 V. A stack of 20
NE2s would therefore require about 1,800 V to start and would run at between
1,200 and 1,500 V depending on current. This is not quite the same as a
HeNe tube (at least for the starting voltage/operating voltage ratio) but
may represent a simple test if you have a drawer full of lonely neon bulbs!

The NE2s can also be used in place of the large resistor described above:

Measure the voltage across Rs to determine current. The sensitivity will be
1 V/mA. Also measure the voltage across the right-most NE2 (IL20 in this
circuit). Operating voltage is then: (75 * V(Rs).)+ (20 * V(IL20).)

This approach can also be used instead of the that using a variable resistor
in the section: Selecting the Ballast
Resistor Using a Dummy Load. Simply insert or remove individual NE2s as
a means of evaluating the power supply's characteristics. However, see the
caution in that section with respect to possible damage to the power supply.

It may be possible to do a very simple test to determine if the linear
regulator is doing anything: Short the output of the power supply through a
DC current meter (e.g., 10 mA full scale for a power supply rated at 6.5 mA)
and power it using a Variac. Over a certain input range (related roughly
to the output voltage compliance range of the power supply), the output
current should remain fairly constant and correspond to the rated current
(e.g., 6.5 mA) or the current adjustment setting (if there is one).
CAUTION: Make sure you start with the Variac all the way down. As
it is increased, the current should increase and level off at the rated
current - only go high enough to confirm regulation - don't push your luck!
How well this works will depend on whether the regulator circuitry is
adequately powered at low line voltage - it may be necessary to add some
resistance in series with the meter (as described above).

Testing to determine if the power supply is producing adequate starting
voltage is trickier since the current is so low and/or the output will be
a short HV pulse (trigger type starting circuits). Any normal VOM or DMM
will look like a short circuit to typical voltage multiplier starting
circuits and/or will not respond to the short pulses of trigger type
starting circuits. They may also be damaged.

Note that ambient conditions can significantly affect the ability of
some power supplies to provide the proper starting voltage. Multiplier
type starting circuits tend to be very high impedance and high humidity
may effectively short them out (especially if the power supply is not
potted or sealed - there may be nothing wrong with either your tube or
power supply!

If you have a high impedance high voltage probe for your VOM or DMM (or
oscilloscope), this may be used to test voltage multiplier or wide
compliance type designs. Use the probe without any load. The voltage
should ramp up to a maximum level determined by the design and ambient
conditions (humidity will reduce it).

If you have a high impedance high voltage probe for your oscilloscope
which has adequate frequency response, use this to test pulse (trigger)
type starting circuits.

Wide compliance type starting circuits may be tested using a normal VOM
or DMM if it has a 10 kV range (most do not without special HV probes).
Even the 5 kV range of a Simpson 260 VOM may be adequate for many smaller
power supplies.

Alternatively, knowing the input impedance of the voltmeter, a high value
resistor can be added to extend its range. See the document:
Simple High
Voltage Probe Design.

Note: Some inverter type power supplies (especially power supply bricks)
use a combination of a voltage multiplier AND medium compliance design so
testing with an instrument that does not have a high impedance input (i.e.,
greater than 250 M ohms) may be misleading (the portion of the starting
voltage produced by the multiplier will be effectively shorted out leading
you to condemn a power supply that is actually good).

A rough estimate of starting voltage can be made by carefully positioning
the wire lead to one end of the load (described above) a fraction of an
inch away from the resistor resulting in a spark gap.

CAUTION: Don't omit the load resistor to limit current - otherwise, the
internal filter capacitor(s) of the supply will discharge rapidly if a
spark jumps the gap and this may be bad for the supply - the high voltage
rectifiers in the starting multiplier and other components may get fried.
Don't ask me how I know. :(

WARNING: Use a well insulated tool (like a plastic stick) to adjust the
spark gap if necessary. There may be 10 kV or more present if your power
supply is working properly and that can bite (especially since if it jumps
to YOU, the charge on the main internal filter capacitor of the supply
won't be far behind!

For dry air, breakdown voltage is about 25 kV per inch. However, many
factors affect this including the shape of the contacts (pointed or
smooth), temperature, humidity, etc. so this will not be a precise
measurement. Setting the gap at about 1/4" will result in a breakdown
voltage of about 5 to 7 kV. There should be sparks periodically (one
every few seconds to several per second depending on the power supply - if
the starting voltage can jump the gap. If there is no evidence of sparks
even when the gap is very small, there is a problem with the starting
circuitry.

Basic testing of a HeNe laser power supply without test equipment can
be performed by simply determining if it will generate enough voltage to
arc a fraction of an inch. This actually only tests for the starting
voltage, but in most cases if the starter is good, the rest of the supply
is good as well. There can be exceptions where either only the starter
works or there is no regulation for the operating current, but it is
still a good first test if no compatible laser tube or head is available.

However, this must be done safely for both you and the power supply:

Obtain 8 to 10 1/4 to 1/2 watt resistors totally about 1M ohm. These
should even be available from Radio Shack. The resistors will limit
the output current to a value which won't damage the supply.

Make two series stacks of the resistors with about half in each
and insert one in the positive output of the power supply and the
other in the negative output. CAUTION: If you recently attempted
to apply power to the power supply, it's output capacitors may
still be charged! Of course, if you accidentally touch the output
and fly across the room, the supply is most likely good. :)

Position the ends of the two resistor strings about 1/8 inch apart for
power supplies used with laser up to 1 mW, and 1/4 inch apart for larger
ones.

Keeping all parts of your body safely away from the resistors, apply
power. Note that higher power units will likely have a "CDRH delay"
which will prevent the supply from actually coming on for a few seconds.

After the delay, if present, a good supply will easily arc across
the gap between the resistor ends.

CAUTION: Do not allow the power supply to run for longer than a few
seconds. Even with the current limiting resistors, it still is probably
not going to be entirely happy driving a load that is not a laser.

If there are no signs of any sparks or arc, try using an insulated tool
(e.g., dry wooden stick) to push them slightly closer together. If still
nothing, check that your input voltage is correct and live, and that there
are no enable signals that need to be applied.

After removing power, use an insulated tool to push the ends of the
resistor strings together to short them for a few seconds. This will
discharge any capacitors inside the power supply and make it safe to
touch the wiring.

CAUTION: Make sure you discharge both the output of the power supply AND the
HeNe tube itself with a high value power resistor (e.g., 100K consisting of a
series string of 5, 20K, 2 W resistors to handle the voltage) before touching
anything. Else, you may be in for a nasty surprise!

Totally dead lab-style power supplies. Check for the interlock
plug and install a replacement if it is missing. WARNING: The interlock
plug usually interrupts the 115/230 VAC power so take care if using a
bare wire! Check for blown fuses. If there is a blown fuse with a brick-type
supply, it is probably dead but worth trying a single new fuse just in case
you got lucky. With a discrete supply (transformer, rectifier/doubler.
filter, HV multiplier), there might have been an overload or short due to
a faulty laser head or its wiring. So, again, try a single new fuse. If
that blows, further troubleshooting will be required.

Totally dead potted power supply brick. Well, these are almost certainly
totally dead permanently. They make nice high-tech paperweights or ballast
for your car in the snow. :( :-) However, make sure you are providing
the proper input including any TTL or other enable signals that may be
required.

There really isn't any way I know of to totally depot these non-destructively
for analysis or repair. However, it is possible to get to the bottom of the
PCB on most units using something like a heat gun along with dental picks and
various other small tools to remove the bubbling Epoxy. See the section:
LITEON Model HA-1170-1 HeNe Laser Power Supply
(LO-1170) and the next section for more discussion of tools and
techniques.

Short of those extreme measures, make sure your wiring is correct and that you
are attempting to operate the supply on the proper input voltage (e.g., 12
VDC, 115/230 VAC - though it may be too late if you guessed wrong and
connected 230 VAC to a 12 VDC unit) and that your HeNe tube and ballast
resistor are appropriate for the power supply's ratings - start with a small
HeNe tube or even just a load resistor and multimeter. See the sections
starting with: Selecting the Ballast
Resistor.

Brick type power supply has exploded. This might happen if a 12 VDC unit
were connected to the 115 VAC line or if no fuse or an oversized fuse was
present and an internal failure caused a capacitor to blow its top. See the
section: Importance of Fusing Power Supply
Bricks. With access to the crater and the bottom of the PCB, it may
be possible to identify the bad components. If fairly localized, the
bad parts could be excavated and replaced. But I'm not sure this would
be recommended.

Portions of a 'brick' type supply can fail but the unit may still be
useful:

Where the regulator has failed (the DC current is not controllable,
except by varying the input voltage), an external regulator can be added or
the supply can just be used on a Variac or with a large enough ballast
resistor to reduce the current to the correct value for your tube.
Current Regulators, and specifically
the section: External Regulator for Inverter
Power Supply.

Line operated supplies are so simple that it is usually possible to locate
failed parts with simple ohmmeter tests or by substitution. Common problems
include failed regulator components, shorted HV diodes or capacitors, and
faults with the power transformer.

The power transformer may develop shorted turns or arc internally. If it
appears totally dead, check for broken wires at the terminals or a blown
thermal protector under the outer layers of insulation. Shorted turns
will result in overheating, reduced output, or total failure.

Electrolytic filter capacitors may dry up and lose capacity resulting in
high ripple which effectively reduces the available operating voltage
since the valleys in the waveform are lower. This is particularly likely
with old line operated power supplies. The result may be erratic behavior
from a power supply and HeNe tube combination that used to work. Make
sure the capacitors are ALL discharged, then test with a capacitance or
ESR meter - or just replace them all .

These types may also develop excessive leakage and reduced capacity
(called 'deforming') from long periods of non-use. This is the same thing
that happens to the large energy storage capacitors of photoflash or laser
flashlamp power supplies when neglected for years. A simple test is to
measure the time constant of the capacitor(s) with a high value resistor.
Usually, you can do this without removing them from the power supply and
just using the normal bleeder resistors that should already be present.
If the discharge time is much shorter than expected, excessive leakage is
the likely cause. It may be possible to revive these by running the power
supply from a Variac, first starting at low input voltage and slowly
increasing it as the capacitors 'reform'.

The transistors in linear regulators may fail if the output is
subject to an arc fault, short circuit, or attempt to drive a tube
with a broken bore. Test each transistor for shorts and opens - dead
ones are usually easily found. It may be best to replace them all
even if only one is found to be bad.

Older Coherent and Hughes supplies are among the few linear types that have
potted regulators. If transistors fail in these, an external regulator can
be installed.

Testing of other components is described below.

Inverter type supplies that are not potted can also usually be repaired
quite easily unless the high frequency transformer has developed an internal
short. However, other component failures are more likely. Where an IC is
used for the control circuit, a datasheet will prove invaluable in analyzing
circuit operation. As with transformers, ICs really don't fail that often
so you really shouldn't suspect these possibly hard-to-find parts without
eliminating other possibilities.

The main switching transistor(s) will likely fail shorted so that simple
multimeter tests will suffice to locate bad parts.

Primary side controllers may malfunction due to defective electrolytic
capacitors, failed pulse width controller IC, or other bad components.

The high frequency transformer may develop shorted turns or internal
arcing. These can often be tested like flyback transformers.

Secondary-side regulators or ripple reducing circuits can fail,
especially due to short circuit faults. Monitor the AC ripple at the
top of the ballast resistor (or into a dummy load if the tube won't
stay lit). It may be possible to add an external replacement.

Testing of other components is describe below.

For components common to all types of power supplies:

High voltage rectifiers generally fail shorted but not always. However,
the forward voltage drop for these is order of .7 V/kV of their PIV rating
so a normal multimeter may not provide enough voltage on the diode check
range to test them for an open failure (which is uncommon anyhow).

High voltage capacitors will generally fail shorted or break down when
stressed at normal operating voltage. Testing for shorts will identify
dead caps but substitution is the only way to be sure unless you find one
split in half or spurting 5 foot flames. :-)

Post regulating components may fail and result in either no regulation or
very reduced output current. Zener diodes and pass transistors can be
tested with a multimeter - these will usually fail shorted. ICs like
LM723s may fail in a variety of ways - if the readings don't make sense,
just try a replacement - they are usually not expensive.

For much more information on the servicing of these types of devices, see the
following (as appropriate for your power supply):

While generally considered to be completely un-repairable, I've found
that it is possible to repair and even completely reverse engineer epoxy
potted bricks using common tools and no nasty chemicals. To get started
I would suggest having the following items on hand: Heat gun (the sort
thing used to shrink heatshrink tubing, strip paint, thaw pipes, etc.),
utility knife or box cutter, various flat blade screwdrivers, multimeter,
soldering equipment, and the patience and curiosity of a cat. Other tools
that are nice but not absolutely required include a set of dental picks,
a putty knife, hot air pencil as used for surface mount rework, fine
point permanent marker, magnifying lamp, bench vise, etc.

Getting to the bottom of the PCB:

The first step is to identify where the PCB is located. Usually the label is
on the top but this is not always the case Look at the end of the brick
where the wires exit. Normally the PCB will be on the long edge opposite
of the wires. If the wires exit near the center look for a current
adjust pot which will usually be mounted directly on the PCB. Failing
that you will just have to start excavating carefully and see what you
find, the PCB will be only a few mm from the surface of the epoxy as
shown in Edge of HeNe Laser Power Supply Brick Showing
Location of PCB and Components.

There are three basic types of casings that I've encountered so far. The
first is a molded plastic box where the PCB has been stuffed down in it
and Epoxy resin poured in, filling the box up to the top. With these you
will have to cut around the perimeter of the area you wish to expose.
Once you have cut all the way around it, apply heat to soften the bond
peel away the plastic layer. The result is shown in
Bottom panel of HeNe Laser Power Supply Brick
Removed. Take care not to use too much heat as many
plastics will release stinky and probably toxic chemicals.

Others have a molded top shell with a panel glued over the bottom after
resin has been poured over the PCB. You may have to cut around the
perimeter of this to separate the glue, then the panel can be removed to
expose the Epoxy. Some will peel off fairly easily, others are very well
attached and may require heat. I've found that a stiff putty knife with
a sharp edge works well to separate these, starting from the edge where
the wires exit.

Lastly, my favorite type uses a separate panel made of thin glass Epoxy
laminate similar to that used to make PCBs which are formed into a box around
the PCB and filled with resin. These are nice because with some care the
panels can be peeled away and glued back on after completing the
repair, leaving a cosmetically nice result with little or no sign that
it has been repaired. Apply heat and start to separate with a utility
knife. Once it starts to come off, a putty knife can be used to finish
the job. Don't get too carried away or you will break the panel and
don't use *too* much heat or it will deform.

Once the Epoxy is exposed comes the fun part. Apply heat to a small
section, holding the heat gun an inch or so away for several seconds.
Don't be afraid to use plenty of heat, but if it starts to smoke, stop!
Quickly before the Epoxy cools, dig in gently with a screwdriver taking
care not to gouge the PCB and damage traces. Once it starts to separate
it's usually pretty easy to work the screwdriver under it and peel it up
in sections. See Start of Removal of Potting Compound
from Under PCB in HeNe Laser Power Supply Brick.
If it starts to harden, apply heat for a few seconds and it
will soften up again. You may wish to expose the entire PCB, but if you
have a good idea of where the problem is located this may not be necessary.
A complete Epoxy-ectomy is shown in Complete Removal of
Potting Compound from Under PCB in HeNe Laser Power Supply Brick.

Replacing defective parts:

There are two options here in many cases. Depending on the layout and
the nature of the failure, it may be possible to simply tack on a
replacement component on the bottom of the PCB. If the defective part is
shorted it will have to be isolated. This is easy on a single sided PCB
but may be much more difficult if double sided.

Personally I like to remove the defective part and replace it properly.
If you are lucky and it's located near the edge of the PCB, you can
excavate into the side and remove it. A hot air pencil is handy here,
but a heat gun will work with some care. Scraps of cardboard, wood, or
other material can be used to mask areas that you don't wish to melt.
Heat the Epoxy to soften and start digging it out with small
screwdrivers or picks. Go slowly and take care not to damage nearby
components. If you can identify the nearby parts, it's worth measuring
the value, otherwise you'll be looking at the mangled remains wishing
you knew what it was. Once the repair is complete there are a number of
options. It can be simply left exposed, but if it is in the high voltage
section this may cause problems, these circuits were laid out assuming
they would be potted in epoxy so spacing between adjacent components may
not be adequate. Silicone caulk or Epoxy work well too, but make sure
whatever you use is appropriate for high voltage. If in doubt, test it.

CAUTION: The HV capacitors in the output section can hold a charge for a
shockingly :( :) long time. Discharge them before touching!

It is quite common to inherit a HeNe laser power supply which once had a
linear regulator but all that is left is either a gaping hole or a bunch of
shorted transistors. :( This may even happen with an AC line operated HeNe
laser power supply using a potted high voltage module. If the regulation
components exploded, there will probably be no output. If they shorted -
probably most common - the current will either be too high with no regulation
or the laser will be flashing due to insufficient ballast resistance. (A
constant current regulator looks like a high resistance to the laser.) While
the purists among us will insist on a faithful reconstruction, for many
purposes, simply replacing the regulator with a few resistors will provide
adequate performance.

Put a current meter set for 10 mA or more full scale in series with the HeNe
tube cathode. Build up your stack from 10K to 25K, 1 to 2 W resistors. Start
with about 50K ohms for each transistor that used to be in series as part of
the regulator. Where the number of transistors is unknown, just start with
50K to 100K. Add or remove resistors to adjust the tube current to its rated
value at normal line voltage. Within about 5 percent (e.g., 0.3 mA at 6.5 mA)
should be chose enough. Err on the low side if the laser seems stable at
this value. If a Variac is available, check for stability with respect to
line voltage variations - the laser shouldn't drop out until the line voltage
drops to below about 105 VAC (in the USA).

WARNING: Make sure your power supply capacitors are fully discharged before
touching anything!

The efficiency of this setup will be exactly equal to that of a proper
regulator. There will just be no regulation with respect to AC line
fluctuations. But with the added resistance of your stack, this won't be
as bad as it would be with just a ballast resistor. Since you presumably
won't be swapping HeNe tubes, regulation with respect to tube characteristics
isn't that important.